If you are running the wire just for lights , i would use 12/2. If you are going to use it for more than lights, then i would use 10/2 which is a little bigger. I would also us a conduit pipe for either wire used. PVC pipe will work also.
20 amp breaker, 12/2 wire , waterproof box and plastic conduit if you are surface mounting it, or just a waterproof face plate if you are flush mounting the box