1- SIZING
2- BRANDS
3- ASKING
Section 1: Sizing
When buying your first bra, or your first bra from a particular store, it is a good idea to get a fitting. In Victoria's Secret and Aerie, two popular stores for teenager bras, they will size you to the correct size. This is an extremely useful tool if you aren't willing to take awhile in the store! Have people help you, don't only rely on your judgement!
Section 2: Brands
The brand of a bra can really make a difference! In sizing, types, and even comfort. I personally prefer Aerie, Victoria's Secret PINK, and Xhilaration (which can be found in target). If you are a size that you know is hard to find, definitely check online!
Section 3: Asking
Asking your parent to purchase your first bra is definitely a big deal! They might be a bit hesitant at first, so offer a compromise! Maybe start off with training bras, or sports bras, or have your parent go with you.