In D&D 4E the experience point system is extremely simple, but they can be confusing if your new to RPGs so I will clarify for you.
I don't currently have the rule books so I can't quote things but I played 4E for 5 years.
The XP for an encounter is added up, then divided up into equal parts per player in your group.
Example: You have 5 players of 1st level so a standard encounter for their level would be 500XP, now we pick some monsters, we'll pick 3 kobold skirmishes (300XP) and two kobold slingers (200XP) for a grand total of 500 XP now your players come in to the room and defeat all the kobolds, since there are 5 players everyone would get 1/5 of the total XP for the encounter, so everyone would get 100XP.
If you have 3 players everyone would each get 1/3 of the total XP for the encounter, so in this case everyone would get 166XP if you have 3 players.
You can also just have your characters level up about every 10 encounters or so where a Hard encounter (2 levels above the party level) counts as two standard encounters and where two easy encounters count as one standard encounter.
This method works better if you don't really like keeping track of XP but you should talk to your players about which method they prefer, some players enjoy getting a reward after every encounter.