First, built the world (at least in your head).
Land: Landmasses are formed by plate tectonics. Some areas are where plates push apart (like the middle of the Atlantic Ocean), whereas others are where plates push together and one ends up on top of the other (such as the Himalayan mountains). Fun fact, the Himalayan mountains are getting taller as the Indian and Asian plates (I think that's their names) push together. Once you have an idea where your land is and where the mountains will be, it's easy to put the rest together.
Water: Water goes from the water table to the ocean. Once its in the ocean/large body of water, it evaporates, becomes a cloud and falls on the world in rain, thereby returning to the ocean. The water table occurs somwehre between the surface (near rivers) to as many as 1200 feet below the surface of the ground. Any time the water table is at ground level, but there is not a river nearby, it's called a spring. Springs produce anywhere from a trickle to a torrent of water (usually I've seen a gallon a minute as a rough average). Once the water comes out of the spring, it goes downhill along the path of least resistance. If the hill slopes at 15 degrees, forward and left, it will go forward/left. If there is a rock in the way, it will go around it, etc. The only exception to this is glaciers. If the planet you are building had an ice-age, large rocks will have been pushed away from the polar region by growing glaciers. However, as glaciers receded the holes left by those large rock formations will then fill with melt-water. Rivers over thousands of years will erode valleys into steeper and deeper canyons, and waterfalls erode the point of the falls, thereby moving upstream slowly.
Climates: Does the world have a hot enough portion to make a tropical zone? Is it cold enough for there to be a polar ice cap? Figure out where the boundaries lie on your world next. The Koppen climate classification could be a good place to get started, but I find it a little too specific for my taste. It looks to measure everything along the lines of average temperature and average rainfall/humidity. Where those two numbers line up gives you a climate zone. Although, cold temperatures cannot support as much atmospheric humidity.
People: Once you have the world's physical features fleshed out, you can work on the impact of humanoid/sentients. People tend to live near water and/or other resources. We have been lazy as a species since the days of the Bablyon Empire (located in the fertile crescent between the Tigris and Euphrates rivers). We also like stuff that isn't readily available. Maybe it's citrus fruit in non-tropical climates or Silk anywhere BUT the Orient. We build roads to connect those resources to where consumers demand them. Once you know where all the luxury goods and trade goods are produced, and the trade roads, anywhere two or more roads converge tends to have a city (or if a road comes close to another intersection, it tends to bend to the city at the intersection and then back along its path). Again, we still like to go the easy way, so we tend to pick the 20 miles of rolling hills to build a road through than the 5 miles of steep mountains. Frequently roads travel along valleys because water tends to collect there in rivers... and merchant trains of antiquity were lazy and didn't want to ride all day, then go find water for the camp. As civilization builds along these trade routes you will build toward there being a small hamlet/village/friendly farmer spaced about as far apart as a day's walk. But if this is a new road or a primitive world it is likely that there are days between villages/towns/cities. Also, where there is not civilization there is opportunity for bandits (or agencies of illegitimate taxation). In a world where there are dragons, I would imagine that most people would also avoid known/suspected lairs of the megafauna that enjoy villager for dinner.
Negative consequences Sometimes people do stupid stuff. Other times they do the wrong thing without realizing they are overtaxing resources. Strip mining, and primitive farming both invite massive amounts of soil erosion due to the tracts of unplanted ground that get left behind. Contemporary farmers always plant a winter-cover crop of grass or some thing similar to hold back erosion of nutrient-rich soil, but they did not know these things back when most fantasy games would take place. Depending on time-frame/genre you also have industrialists who will spill resources (such as oil) and ruin otherwise good ground just because it's too easy to leave it there and let someone else clean it up. Another common problem is over-using resources. I read an article about China's efforts to prevent desertification near Beijing so there wouldn't be sandstorms during the Olympics. Overusing/diverting water causes forests to become grassland and grassland to become desert. Similarly, with enough irrigation deserts become grassland and grasslands become forest.
Finally, to put this tempest back in the teapot: Take your time and draw it the way you see it in your head. Talent is not required, I suck at drawing but the one week I forgot to bring the continent map to game when I DM'd, the lack was noticed. I would recommend you draw the continent map on something the size of standard paper (letter/A4/whatever it is in your location) and keep it to just pencils or other easily erased media. Most of my sketches like this are done on graph paper because it helps me when I need to draw symmetrical things. Another trick is to have a color-coded map. I start with the USGS convention (green is vegetation, blue is water, brown is terrain, black/red is human activity) and then add other colors as needed by the system (yellow is partial concealment, orange is full, etc.)
I try to plan likely encounters (or plot encounters) ahead of time so I have the sketches ready to spin up a battlemat quickly, but random encounters are just as fast for me... typically draw a road, where the trees/hazards are, and have the players place their minis as necessary. If you don't do mats, have the sketch and then put each character's initial in a square for where the character is on the grid. Either which way, if you are sketching where Tucker's Kobolds are attacking the party, you don't really need to know all the geo-political implications... just where the location and direction of travel for both the PCs, then NPCs and the proximity of either to local hazards. You don't need to draw all that, but it helps knowing that if your party is travelling through the Fire Swamp to put a few Fire springs on the map for added DM fun (or only mark them in your mind until an unlucky PC steps on one and gets singed).
One final note: In homebrewing circles there's time-old advice of "Relax! Don't worry! Have a homebrew." I'll co opt it here to "Relax! Don't worry! Have fun!" As long as everyone is having fun, any mistake that you make by hastily sketching a map should be overlooked by most reasonable players, so tell your inner-critic to shut up and play when it starts heckling you. Any discrepancy fix and say "oops"... Or provide a reason the river runs uphill (if it's a fantasy setting).