There's a lot of confusion and stupidity (on the part of manufacturers) but here's the basic:
Measure just under your breasts, this is your band size. Then, measure the fullest part of your breast. If you're 1 inch bigger, it's an A cup, 2" is B, 3 is C, etc.
I know there's a lot of 'measurement plus 4 is your band, etc but that doesn't really work.
Good luck.
Oh, and if you need to buy something sight unseen (which is almost impossible with bras) I can tell you that vanity fair does fit me according to those measurements (usually LOL).