How can i make levels to a fps game

How do i make levels for my fps game like: I go shoot a NPC or Player and get some exp and when i get like 1000 exp i get a level, and then when i got to a higher level i need to get more and more exp the higher level i am? like call of duty modern warfare 2.

Thanks.

There are a number of topics already on this problem: http://answers.unity3d.com/search?q=experience