So i recently embarked on a project to begin a kind of 3D sidescroller a bit like the modern mario games.

Im not so much having issues, infact its going really well, however, i was wondering, how would i put collision detection into the game so that the player falls where there is no floor and doesn't when there is floor.

Thanks a bunch, if you need the code just ask

You could also use Phys2D/Fizzy/JBox2D if you only need physics in two dimensions, i.e. like Mario games.