You're.. developing. It's awkward. You think it's awful. I'm here to help you out.
You see from here (11 years later), I'm looking back on your mortification. You think it's uncomfortable wearing a bra and you HATE the fact that you have to start wearing one. How are you going to play football at recess now?
But let me tell you: soon enough, you'll feel comfortable in your own skin again (and it's sooner than 11 years). It's going to be awkward at first, but you'll get the hang of the clasp in the back. Bra shopping really sucks-- so leave it to Mom for as long as you can. Then think of it as joining a new club, where more than half of the population is right there with you.
And in the mean time, don't wear a white bra with a white shirt. Trust me.