When your breasts begin to jiggle, feel sensitive when you run or feel heavy, it's time to consider a bra. The best advice for choosing a bra is to get one that fits and is made of comfortable fabric. A bra is right next to your skin all day long, so if it's a little scratchy or tight when you try it on in the store, it will drive you crazy by the end of the school day.

To determine your size, use a tape measure to measure around your chest just under your breasts. If the number is even, add four inches to get your chest size. If you measure 28, for example, add four and look for bras in size 32. If the number you measure is odd, add five to get your size. So if you're 29 inches, you'll look for bras in size 34. Bras typically come in chest sizes ranging from 28 to 44, with larger sizes available at specialty stores and through catalogs.

Next you need to determine your cup size, or the size of your breasts. Cup sizes range from AAA (very small breasts) to EE (very large breasts). Measure around your chest, this time lining up the tape with your nipples. If the measurement is the same as the first one you took, your cup size is AA or AAA. If it's one inch larger, your size is A; two inches larger, size B; three inches, size C; four inches, D; five or more, DD or EE.

The measurements will give you some guidance, but in the real world bras marked the same size vary considerably, so you may have to try on several sizes and styles to determine which is best; the fit should be smooth and not too tight. Do not buy a bra that puckers or squeezes your breasts, leaving bulges of flesh poking out around the sides. A saleswoman can help you find a bra that fits.