Problem with CharacterController variable isGrounded

I have a character, and I want to make it jump. However, I only want it to be able to jump if it is touching the ground (no double jumps). I want to do this using the isGrounded variable, but how do I make something that qualifies as "ground"? I have my character on a simple sprite image of the ground with a huge mass so that the ground isn't pushed by the character, which is affected by gravity (the ground is not). What do I need to do to make ground that triggers the isGrounded function on my character?

 

Tags will work for this. Just create a tag for the ground, apply it to anything that is grounded, and when the isGrounded object comes into contact with anything with that tag it will return true. I believe you could also just use layers here, using the same method. Search through Unity Answers as it has many solutions to quick problems like this