Could anyone suggest me how to do bullet detection?

I am currently working on the first person shooter.
Everthing works fine…now comes to the stage that to implement bullet detection. As you guys know, thats impossible to turn 2D inputs into 3D coordinates. So, what could I do?
I have been searching on Google. I cant find any suitable web site for reference. I would be very appreciate if anyone could help me! Superb thanks!

no need to cross-post on the wrong forums.

very thanks
sorry about that
i wont do it again