Put on a non-padded bra that fits before you start shopping either on Victoria's Secret website or in one of the stores.
Wrap a flexible measuring tape around your back at the band of your bra, up to the front above your bust. Take a look at what you have as a measurement -- this is what's considered your band size. This number should be an even one, so if your measurement turns up an odd number then round it down to the nearest even one. Write down the number because you'll be using it later.
Check your actual bust measurement by wrapping the measuring tape around your chest across your nipples, which should be the fullest part of your breasts. Don't pull the tape too tight so that you're squeezing the girls; the tape should instead feel comfortably snug, but not binding or so loose that it's sagging. Look at your bust measurement and if it falls on the half-inch, round the number up.
Figure out your cup size by subtracting the size of your band from the measurement that you got for your bust. The number that you will get is what will help you to determine what cup size you should wear when buying your Victoria's Secret bra. For example, if the difference is 0 inches your cup size is a AA, if the difference is 2 inches your cup size is a B and so on down to 6 inches which signifies a DDD cup.
Use this measurement when buying your bras from Victoria's Secret. If you are shopping in person and feel shocked (or maybe a tad disappointed) that your bra size is bigger or smaller than what you've always thought it was, ask the salesperson for a complimentary fitting to just to make sure you got it right. She should be able to direct you to the appropriate size bras that will fit you according to either your or her measurements.
Click "Search Bras by Size" on the Victoria's Secret website if the Internet is how you roll when it comes to shopping. This way you'll quickly find your size once you've narrowed down your choices by selecting a band and cup size based off the measurements that you've taken from home. Select the your size from the drop-down box and then your bra style of choice to start shopping for that gorgeous new bra.