HitTest/ Collison Detection in 3D?

hi everyone

i’m really stumped on how to implement some hitTesting or collision detection in this 3D environment.

its based off some of the tutorials from here but modified a little to suit my needs.
anyway, the style is supposed to be something like Diablo or Fallout, where the player walks around viewed from above (almost), but can also walk behind things and i need the player to be able to collide with these objects, be they builings, items, or people. but more important than just testing to see if they hit them, how can it stop the player moving when walking into a wall for example.

if anyone can help i’ll give you a special credit when it’s finished (it’s part of my university course)

thanks
Booeriis