When I needed my first bras, my mother brought me to a store where they actually fit you (a woman goes into the dressing room with you and helps you find ones that really fit. They are specifically trained for this - it's an art!) so if you can find a place like this, it's important that her bras be comfortable and the correct size.
I would also recommend soft, 100% cotton bras - they don't make you sweat, and no underwires which also make you sweat and cause rub marks. I quit using underwires (I've got 36C) about 6 years ago and it was the most comfortable decision I've ever made about clothing!
Early intervention specialist and parent consultant since 2002.