The single best shot any new league player can learn is a dependable second serve. The serve is the easiest stroke to practice on your own, and it is the shot that gives most rec players the most frustration.
I would practice the snot out of the kicker. Once you have a second serve that you can count on to get in and not be attacked then you take the double fault out of the picture. Everything else loosens up for you after that.
As an aside, I often use the kicker for my first serve.
"You should be playing linebacker, not singles."