Go to Macy's or Victoria Secret or another reputable place to get correctly measured for your bra size. A high percentage of women wear the wrong size. Call ahead to make sure there is someone there trained to measure you. Buy one bra there. Then, once you are correctly sized, you can go anywhere for bras.
Keep in mind that Victoria Secret Bra's IMHO tend to run slightly smaller than other brands.
Once I find a bra I love, I buy alot of them; also, before they have gone thru the wash too often and the info on the label fades, write down all the information about the brand (size, style #, brand) so you can reorder, perhaps less expensively, on the internet
Use a lingerie bag to wash and air dry bras so they don't get stretched.
Hope this helps