I never knew what a sanitary belt is until i just googled it oO. Where i live the pads have double-sided tape to hold them in place....
back on topic: since you're quite sure you'll need one, you can just go to a store by yourself, and take the smallest five sizes to the fitting room.
another option would be to measure yourself before you go bra shopping. there's bra size generators on the web. all you need to do is measure around the widest part of your boobs and then measure the circumference right below your boobs (the latter gives you the size as in 32 inches, 34 inches etc.). The cup size (A, B, C..) depends on the difference between the two numbers (e.g. 36 inches - 34 inches).
Band inch 0 <1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Cupsize U.S. AA A B C D DD DDD/E/F DDDD/F/G G/H/J I/J/K J K L M N - - -
Cupsize UK/Austr. A AA B C D DD E F FF G GG H HH J JJ K KK L
.....once you have found your size, e.g. 32 A, you have a rough idea of what size to buy. unfortunately most bras run small or run large, so if you measured yourself as a 32 A, you should also try on a 30 A, 32 AA, 32 B, 34 AA and 34 A. I know, it's a hassle, but you want your first bra to fit.
I'd go for a place like target rather than victoria's secret etc. because they carry more different sizes (for girls in their early teens and such) and shapes.
For your first bra you might want to get something that's mostly cotton and maybe doesn't have an underwire. At least that's what's recommended for young teens.
A sports bra does keep them down a little bit, but they're kinda hard to find in small sizes. maybe a teen sports apparel department would be a place to find one - or else, H&M carries XS sports bras.