Collision If Statement And Falling throught the ground (2 questions)

Is there a way to make a if statement for collisions? if so what is is? Or can You give me a link to a tutorial?

I would think it would be something along the lines of rigidbody collisons like:

 if (player.rigidbody.collisionWith(object)){
 //Do stuff here!
 }

or something but i can't seem to put a rigidbody on my character because i always fall through the ground. I have generated colliders on my character i tried meshcollider and character controller but i just float in the air. I have made sure i have terrain colliders on my terrains. That would also be nice if someone could answer that :D

Thanks In Advance.