Help with collisions.

I need some help with simple collisions; I don’t know how to write them. Here’s what I want to happen.

Check the type of collision (which weapon was shot)
React according to distance and strength of the bad guy or good guy
Return to normal state after Recovery

Thanks to all who help on advance. I use DJGPP or VC ++ depending on which mood I'm in, happy or sad.

if your using opengl i suggest the plane collision tutorial at www.gametutorials.com, its very good

Thanks.