You cannot create a game on your own, unless you have absolutely no life, all the free time in the world, a brain that won't explode, and an unimmagenable amount of dedication.
Though you can make a game if you assemble a working team. You would need:
-Coders
-Modelers
-Mappers
-Sound Artists
-Spriters
Also you would need an engine to base your game on, HL, Quake, etc.
Also, assembling a team to do this is not an easy task depending on what your basing your game on.
You can find out alot about making games if you search on google and read articles on proper mod making.
Hope this helps...