Start with a great posture. Stand up straight, pull your stomach in then lift your shoulders, roll them back and drop them down - repeat when ever you notice yourself slouching.
You can't just work your back you need to balance it by working your front as well - chest and core.
Look on youtube for excercise videos and tips. Try searching core and back workouts and arm workouts. There is a core exercise for boxers video that is awesome. You won't be able to do the whole thing but if you keep trying you will eventually.

Push-ups
Tri-cep dips
Mountain climbers
Planks
Pull-ups (start by swinging your way across the monkey bars at the playground then do a pull-up - playgounds are great to use as exercise equipment )
I don't know what they are called but you lie on your stomach and lift your arms and your legs (alternately then together)