Squats, a lot of them with a lower amount of weight. Like 135 lbs-10 times, and do that for about 4 sets. Once you do one set do step ups onto a box or a chair for about a minute. Then jump onto that box 5 times as fast as you can. Rinse and repeat 4 times.
I can't dunk because I'm like 5'6" and 225, but I know that squats are considered the "master" lift, and you want quick muscles to be able to jump higher so doing more reps and less weight will help make quick muscles and not bulky muscles.