What gauge wire do I use for a 20 amp breaker

A 20 amp breaker is generally used with 12ga wire. You may use this breaker for a larger wire, 10, for example. The purpose of a breaker or fuse is to protect the wiring.

Are you using copper or aluminum wire?

240.4(D)(3) sets the amperage on a 12AWG on AL. wire to 15 amps.

240.4(D)(4) on copper wire to 20 amps.

If you are using copper wire it would be 12 AWG, however if you are going a longer distance you may want to use 10 AWG. If you are using aluminum, that is a different story, so double check.