How do I make it so my character doesn't fall through the floor?

I'm COMPLETELY NEW TO UNITY and I'm trying to make my character in a 2D game stop from falling through the floor. Here's what it currently looks like:


Whenever I press start the rigidbody on the character which makes it fall through the floor.
What collider should I use, and how do I rig it?
Or should I use a collider at all?

 

You definitely need some sort of collider to provide the physics check to see if it should collide with the floor. I'm not really a 2d person, but some sort of 2d collider that approximates the shape of your character is probably best. Maybe try BoxCollider2D as a start?

Or use a character controller - that would include movement stuff and the collider in one thing: https://unity3d.com/learn/tutorials/topics/2d-game-creation/2d-character-controllers