@umbra21 - If in doubt go to a good bra store and get measured. Most higher-end department stores will do this as well. Don't settle for someone who doesn't seem to know what they are doing, or is trying to sell you something expensive.
A good bra fitting can make all the difference between good posture, or bad posture and can prevent you from suffering from back pain or embarrassment from bra malfunction.
Once you've got the sizes figured out, regularly revisit them, because you will change over time. Your bras also simply won't last all that long and need to be updated. That's why they have multiple catches in the back. It's so that you can tighten the band as it gradually wears out.
Finally, you might end up needing different sizes for different kinds of bra. A strapless bra, for example, might possibly need a tighter band size and a shelf bra might need a larger cup size. Trying on your bra, underneath the clothing you intend to wear with it, is essential.