Walk throw the walls

I wright a First Persen Shooter. How can I determine, that there is a door. I want don’t walk throught it.

Please, tell me, where can I read some material on this topic.

It’s called: collision detection. Search google for this. Look at http://nehe.gamedev.net and www.gametutorials.com.

Great thanks, I got answers on my question on tutorials.com