I recently started GMing for my D&D group, and it was my first time as GM. My group often tries to rush through things before I can describe their environment ( ex: running through a cave before I can say "there are 3 goblins blocking your path"). I don't know haw I should discourage them from rushing and interrupting me so much. I have brought it up with them but they continue to do it. How should I discourage them from doing this?
Step 1: Talk to them
If this is bothering you, bring it up at the table. Just explain that you'd like to finish describing the scene and they're cutting you off. Maybe you can work it out without having to do anything else. Remember that your goal as a GM is for the players to have fun, but you should be having fun too. If you're not having fun because of this, it's an issue to discuss with them.
Step 2: Be Patient
Are these new players? Because they sound pretty excited, which is good! But experienced players don't tend to rush so quickly, because sooner or later those "3 goblins blocking the path" are going to be "a wall of fire" or "a giant pit trap" or "a sleeping dragon who is rather unhappy you woke him up".
Eventually not looking around before leaping in is going to get them into trouble. You will then simply have to not let them backtrack on what they did, and watch them fight out of it. That can lead to some memorable moments, and be a teaching experience all on its own.
Step 3: Don't let them
If you don't want to do that (or it doesn't work), your last option is simply to not let them. When they say "I run in!", just tell them they can't do anything until you finish your description, and keep going. This will likely annoy them, though. It's usually better to let their excitement get them into trouble on its own, rather then you reacting negatively to it.
The simplest method is to simply go along with brutal consequences: if they don't wait to see what you describe, then they crash into it headlong. This can be both effective and hilarious when used judiciously. But it does require some care: you don't want to use it when the consequences will be lethal, because that just isn't fun for anyone involved. You want the party to be annoyed, not dead.
Good: "OK. The first one of you through the door trips over a goblin who was standing not far behind it, and the second one of you trips over the first. Your speed bump's buddies turn toward the party, and they don't look happy. Both draw weapons, and if the muffled swearing from underneath you is any indication, the other one wants to also. Roll initiative; two of you are starting prone, and you're pinning the goblin you tripped over."
Bad: "There was a sphere of annihilation in the middle of the room. You die."
Other than that, a simple "hold on" when the players start to interrupt you is generally effective. It's just urgent enough to hold the players' interest for a few seconds (which should be all you need for an initial description), without going overly melodramatic. It may take a while for the players to get the picture, but they'll learn.
The easiest way to deal with this is to ask a simple question: "Are you sure?"
This puts the ball in your players' court. If they reply affirmatively, proceed with whatever you had in front of them. If they run past enemies, the enemies get attacks with advantage. If there was a pit, they fall in the pit. If there was a giant flaming sphere of DOOM they die.
Asking if they are sure will force most players to stop and think about what they are doing. If it doesn't, the only thing that will is for the scenario to play out as realistically as possible- without you purposefully removing threats.
First of all, consider yourself somewhat lucky. Most groups I've played with have the opposite problem: they are so overly cautious it might take 10 minutes to get through a door after all the precautions are taken.
Your players are giving you hints about the pace they want. I would recommend tailoring your descriptions to make them shorter, and to put the most relevant information up front. Also, it might help to write things down to avoid pauses until you get better at winging it. Use phrasing like, "3 goblins block the entrance to an enormous cave," rather than, "You come to a large entrance...it's to a cave with...stalactites hanging almost to the floor...there are 3 goblins blocking the entrance."
As per the other answers, you can give small in-game consequences, but I would make this a last resort compared to working on your pacing and phrasing.
Although Tridus's answer covers most of the ways I would suggest, there is one additional possibility I can think of. If they insist on rushing in then give them in-game consequences to their actions.
For example, rushing into a room could leave them unprepared for traps, enemies, whatever situation you've devised for them. After a couple of experiences where their actions put their characters in a lot of dangerous trouble, you may well find that is enough to get them to slow down a bit.
These are all great answers, but I'd like to address the personality issue. Worst case scenario - you have a group with players that interrupt not only you, but each other and perhaps at least one or two that tend to dominate the conversations. In the best case, the players will work this out themselves and learn to be courteous and wait their turn. In the worst case, you probably need to implement a tool, like running every encounter as a combat round with turns and each person has to wait until their turn, or even a "conch", a physical, tangible item at the table that gets passed around, and only that person can speak. In the end, players like that who do not learn to be more cooperative don't typically get invited back. I'm a sucker for loyalty though, so I tend to try and work with someone that's interested in the game and shows up, sometimes to a fault, losing less patient players.
One way you can do this is by giving them warning that something's inside before telling them the cave is even there. It piques their attention and puts the emphasis on story and challenge rather than location.
"As you approach, a pulsing, fiery warmth washes over you. It carries the smell of metal and singed rock."
"Chattering, indistinct voices draw your attention towards the mouth of a large cave."
"Up ahead, the fog seems to swirl of its own accord. You glimpse unknowable shapes forming and dissolving in its depths, breaking against a series of large stalactites. Roll for perception... [somebody passes] Eyes narrowed, you can make out the shape of a cave mouth, ahead and above you. [if they roll high] Either some foul breeze is stirring the fog and raising goosebumps on your skin, or the mist itself crawls from within. Either way, if there's another way forward, it's hidden in the writhing miasma."
That sort of thing.
One player-GM conflict that seems to come up not infrequently is that GMs want to go through the entire description of an area, despite the players not being interested; GMs will then sometimes bury the lead and leave those goblins to the last so the players can't jump ahead. One of the first thing the PCs see will be the goblins, and thus one of the first things the players hear about should be the goblins. If they're jumping forward despite that, they're probably the problem. If you are burying the lead, and trying to get in lines of description before the goblin, that's probably the problem there.
If it's one player, you may need to work with that player. But if everyone is getting bored during your descriptions, that's probably just not the style of play they like.
Impulsive players is, I have to say, not the absolute worst problem that you as a DM can have. In fact, I'd say the biggest issue is the opposite: PCs who have no sense of agency, who have to be virtually tossed out of bed and into a dungeon. There are a lot of things you can do to players to make them less impulsive, but I think that in doing so you do run the risk of making them too scared to go outside (the old skool dungeon called Tomb of Horrors is a great example of something no good DM should ever, ever lay upon his charges).
That being said, one of the kind of fun things that I like to do as a GM is play the world while the characters move through it. If they're too brazen to take the hints of someone you're trying to implant a story through, go ahead and try to roleplay how that person might react to, for instance, some 1st level fighter talking all kinds of smack about them. Maybe they get angry, maybe they laugh in the fighter's face. Along the same lines, if the characters break the law, you can show them how medieval punishment works (maybe some roughing up by the guards and some time in the stocks if the crime was relatively minor, maybe a tersely worded admonition to get out of town and don't think about coming back if it wasn't) and let them decide if they really want to keep on doing things without thinking them through all the way.
I know that as a player I have the tendency to seek out conflict. Sometimes that looks like I'm being impulsive but dammit, when you put a giant yellow button in the middle of a panel with a flashing light that says "DO NOT PUSH THIS BUTTON", my character is going to find a way to push it. To some degree, RPing is about living vicariously through people who can run and jump higher than we can and/or cast spells. Part of that vicariousness is trying things out that sometimes don't make a lot of sense at first blush, or (if you're me), getting into character to the extent that a stubbon dwarf isn't merely stubborn when the plot asks him to be stubborn but is stubborn even when people would dearly like him to stop being that way.
The one thing I think you want to draw the line on is your own sanity. While I don't think a GM ought to require that the players get on rails and woosh through their meticulously planned campaign step by excruciating step (granted, some GMs do think that way), neither should you as a GM feel obligated to play, for instance, a world around a party of Chaotic Stupid players/PCs. If that's the kind of "rushing" you're talking about, sit down and talk to folks about it (use "I" statements, etc.). If folks don't want to do this, maybe let someone else take a turn at GM, or try playing something else that's more amenable to folks' playing styles and enjoyment. It is, after all, a game, and games are meant to be fun for everybody.
If the description they're trying to rush past mentions some enemy they're most likely going to have to fight, one simple solution is the indicate that immediately, for instance by saying "Combat!" or "Roll initiative!" Once they know they're in a fight, they're likely to be more attentive to your description of what they're in a fight with - if only so they can tell you which enemy they want to target.
Of course, that only works if there's a shift between two modes of play happening - if there's no reason to roll initiative because all they're up against is a wall of fire, then just start telling them the consequences of their actions. If you start with how much damage they take, you'll have their attention fairly quickly - and you can always let them take back their action when they consider that their characters wouldn't have done something so stupid.
As suggested by some colleagues, it may be fun and educative to "harm one to teach hundred". What harm means, depends on you but personally I would choose a climax: first time it may happens something funny, second one someone will get in troubles, third one some will get in REAL troubles.
As side note, I can suggest you to ask your players if you are too verbose or if you tend to get too boring while describing your scenarios.
Have nice fun.
I find that using signals or patterns in my descriptions help immensely. For example, it's a custom at one of my tables to raise a hand when speaking- This means the players can clearly see when the DM is done with his description, and don't rush off to do something before I'm done talking. At another table, we picked up the habit of ritual phrases from Dogs in the Vineyard, and added in "That's all you know" at the end of the DM describing a scene or "that's how it went" when anyone finishes an action. At my FATE table, we have a pet rock we pass around to show who's turn it is. Each table has its own little pattern, but having a clear indicator of who has the floor can be very useful.
If you don't like the formality implied above, or if changing the pattern of your play would be disruptive, I might suggest altering how you describe the scene. It sounds like you're trying to describe things depth first- Give the cave in detail, and then moving on to the goblins huddled in the middle of it. Try switching to breadth first descriptions- "The cave is large, perhaps thirty feet wide. You see a flickering fire, a bunch of huddled goblins, a large chest to the side, and scorch marks on the back wall." Then either do a second pass ("The floor and walls of the cave are worn granite...") or just pause after giving a quick inventory of the area and wait for player questions. I find that it's trickier to convey mood or atmosphere when doing the quick inventory, but it should prevent interruptions. (If your players interrupt before you finish the first sentence, you need to have a talk with them.)
You can convey mood through the questions your players should ask, if that's something you're interested in. (Q. How many goblins are there? A. There are ten crowded closest to the dim light of the fire, but you can see uncountable eyes reflecting the flickering firelight from the darkness of the shadows.) Since these are questions the players have asked, they will be more interested in details than if you begin just telling them. As a side effect, I've noticed that deliberately giving less details than I know the players want (for example, saying "A lot of goblins" without giving a number) will usually get them to ask for details, and in the process of asking for more and more details, they slow down a little as they get a better picture, and they force me to think of things I didn't plan sometimes. (What's fueling the fire? How smooth is the floor?)