You definetly need to get measured. Any of the major deparment stores, (JC Penny, Macy's, Sears), have associates who will help you. Next comes the worst part, try on lots of different styles. In the fitting room, bounce up and down, twist and turn, and get a feel how it will work with your day to day lifestyle. Only buy one of each style. After trying it out for a few weeks at home, wait for a sale or buy on-line to stock up.
If your a C cup or larger you'll want a bra that has at least 3 hooks in the back for better support.
Personally, I like to go to the Hanes, Bali outlet if you have one near you. I've always found that playtex to be a good brand. For online shopping www.jms.com.