Install the breaker as Donald has suggested. Use #12 wire for 20 amps. It can be assorted colors, but typically a black, red, white and ground are used. The black and red are hot, white is the return and solid copper is the ground.
When you punch out the KO in the bottom or top of the panel, you will need a romex connector or a conduit connector. No wire can pass through an opening without a protective coupling (same at the outlet box). If the panel has all holes used, rent or buy a KO tool. It is a sleeve which pulls a hardened plug into a hardened collar after drilling a pilot hole.
Buy a couple of feet more wire than you need. Leave enough slack to move things around. If outside, use UV protected wire, but conduit is better. If inside, plain old Romex is fine. Use clamps or staples to support the run.
P.S. Disclaimer: I'm not an electrician. I have never played one on T.V. You should have a licensed electrician do the installation and have a building permit to run the circuit or resale may become a problem at a later date.
P.P.S. It is better to seek gorgiveness than permission.