I agree with the other girls, get fitted for the best fit. But til then just know if your boobs spill out on the side or in the front or its so snug it creates back fat, not the best fit. Also the straps shouldn't fall off your shoulders every movement and shouldn't leave red marks either. There's plenty of books and websites that show you how to fit yourself as well. You have to measure right in the middle of your boobs all the way around or something to that nature. Its in an old jcpennys catalog I have lol.
If it was me, next time you go to the mall, stop in Victoria's Secret and get a free bra fit :) They'll measure you and help you find a bra your size. OTHERWISE just YouTube "How to find my bra size" and click the first video
One rule I keep hearing- if your back strap can stretch enough for your whole hand to fit through while you're wearing it, or your shoulder straps can stretch more than an inch or two then the bra is too big. Sometimes can be fixed by adjusting straps, but sometimes you need a smaller size.
Your bra should feel firm once it's on, and the cups should have a firm hold on your breasts.