It's good you're starting somewhat small. Check out the codebase for examples of first person shooters. Probably the best way to start any program is with planning. You will need to plan:
1. The story/objective
2. Controls (jump, walk back&forth, strafing, turning, mouselooking, can you customize them?, etc.)
3. All of the levels, or only the one if you just add more guys as you progress
4. Spawn points for player & baddies
5. AI------this is difficult
For AI you need to think exactly like a computer. If your arena is made up of simple obstacles you can probably get away with simple AI.
ie (here's the AI's "thought process"):
Check State
If State=SpottedPlayer Then shoot him
if obstacle is in the way, move around the obstacle
If State=can'tfindPlayer Then walk around like an idiot
If gun is out of ammo, then reload
If you are getting shot at run away and hide
That's very basic, and will get you some pretty dumb AI, but it's a basic idea. Check the codebase.
http://thegamecreators.com/?m=codebase_list
Crazy Donut Productions, Current Project: KillZone
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