fall through ground problem

OK I'm really new to this and it might be my newness or something but this has been quite upsetting

every time i start a new project and put a cube a ground and then put a cube give it a rigid body and play it all it does is fall right through the ground and when i put the old first person controller it just floats and doesn't move at all this has been quite perplexing lately because I've been trying to make a ragdoll with the robot that comes with the fps tutorial and it just falls through the ground i would like to know exactly how to fix this so i can do this in the future and not make the same mistakes