From what I've seen, most women (yes, women, not girls) don't know how to buy a bra! Not only are they usually pricey, but you'll be wearing them almost 24/7 too. You may not notice it, but bras do make a difference in your appearance (so many things could go wrong). I worked at a lingerie store once, so here are some tips that I want to share:
1) Ask the lady for help.
There is no shame in asking! It's the lady's job after all, plus she is a lady, so what you have, she either probably has too, or sees it all the time. It is really important for her to measure you, because #1 different brands and even designs may fit you differently #2 our body changes without us knowing sometimes, so you may have gone up or down a size.
2) Try more than one size.
You may think it's silly but you'd be surprised how different one size up/down feels. It shouldn't be too tight or loose (obviously). If in doubt, ask the lady.
3) Put your top on after trying on the bra in a brightly-lit fitting room.
The clingier and lighter-colored the top is, the better. That way you can avoid buying a bra that makes you look like this.
4) Don't forget to check the back and sides.
You'd think that women remember to, but surprisingly, it happens more often than we'd like to admit. When I shopped for bras in the past, I just wanted to get it over with, which meant I didn't check my back. Needless to say, I regretted it.
You don't want fat squashing out; it's quite unsightly (no pictures!). A wider bra band tends to be comfier (esp. for women with heavy busts), but it's usually not necessary. Just get it in the right size (once again, it's so important to get measured!). In case you didn't know, (using 32A as an example), A is the cup size and 32 is the size of the band. So yes, you can get your cup size with a longer bra band. And if needed, you can easily get an extension.
5) Get at least one in your skin color.
Contrary to what most women think, white bras do show when you're wearing a white top. They also show in black tops (depending on the thickness of fabric), and they do a good job at that too. I used to avoid them like the plague when I was a teen; they look so old-fashioned, I know, but they can be worn with any colored top. Anyway, look, proof!