The band size is the circumference of the bra as it goes around your rib cage. The right band size makes all the difference in finding the right bra, because band size also determines the rest of the proportions on the bra. Measure your band size by using a tape measure -- wrap it around your body just under your bust, keeping it parallel to the floor as you do so. Round this measurement to the nearest inch; if it's an odd number, add 5 more inches. If it's even, add 4 more inches. This is your band size.
Measure across the fullest part of your bust to determine your bust size. To do this, start by wearing a bra that lifts and supports your bust without adding a lot of extra padding; this will help you get the right measurement. Wrap a tape measure around your chest, going across the center of your back and in the front around the largest part of your bust; generally this means it will wrap around your body at nipple level. The tape measure should meet itself in the front, so it makes a full circle that doesn't squeeze your body at all. Determine this measurement and round up or down to the nearest inch.
Your cup size takes a little bit of math to figure out. It's important to know that not all cups are created equally: a 32A, for instance, is a much smaller cup than a 38A. The band size determines the size of the cup as well, so you need accurate measurements all the way around. To determine your cup size, subtract your band size from your bust size. If the number you get is 1, you're an A cup. A B cup is 2, C is 3 and D is 4. Higher than D and you'll need to determine the actual size progression for individual bra manufacturers.
Put your band size together with your cup size and you have your bra size. For instance, you might be a 34B or a 38D. However, no two bra manufacturers make their bras in exactly the same way. Just as with other pieces in your wardrobe, you may be a slightly different size based on the style and brand of bra you buy. Use your size as a guideline when you shop, but be willing to go up or down a cup or band size here and there to find the bra that fits and flatters your figure.