Learn to kick. It will probably take a while. After you learn how to throw a decent kick, you need to work on your power, so I recommend throwing about 15 kicks using each leg so you can develop your power.
Work on your combos, The bag is excellent for working on your speed and power, try and learn some basic combinations, then you can have fun mixing it up a little, throw some elbows in etc... you can also learn how to mix kicks into your combos as well using the bag, if you just learned how to kick chances are you're not going to be using kicks in combos when sparring.
Above all, push your limits but don't break your bodies boundaries. when i was younger I broke my knuckles on my bag so many times that they're deformed now. You cna make the argument that it makes them tougher, but I just think it makes them easier to break.