collision

For your main character entity, use
EntityType char, 1
EntityRadius char, 1

For your world mesh, use
EntityType world, 2

Then, enable collisions with
Collisions 1, 2, 2, 4

And in your main loop, use
UpdateWorld
RenderWorld

Blitz3d collisions are allways sphere-to-something, so that is why you must set the radius for your character mesh.