Lingerie to your outfit is like primer is to paintings. If you're wearing the wrong underwear, no outfit you wear is going to look good!
The things to consider are: FIT, SHAPE and STYLE
FIT
Bra size are measured by band size and cup size.
Band size measures the circumference around your ribs underneath your breasts. For example, if you measure 34 inches around, then your band size is 34. Easy! If you're in between sizes, go up a size. There are usually bra extenders in the back that you can tighten if you're a bit smaller.
Cup size measures the difference between the the circumference around your body at the fullest part of your breast, compared to your band size.
1 inch difference: A cup
2 inch difference: B cup
3 inch difference: C cup
4 inch difference: D cup
5 inch difference: DD cup
Sizing above this is unfortunately not done consistently between brands (watch this space - I will do a full post on this later)
And that's it! You put them together and that's your bra size. If you went up a band size (as you were between sizes), then take a cup size down.
Most importantly - always try the bra before you buy. Some brands size larger or smaller than average. Watch this space - I will be doing some research on how true to size each brand is, and will come back with specific guidance for each brand :)
SHAPE
Shape has to do with what your breasts to look like. This is very dependent on the shape and size of your breasts.
If you want to accentuate your breasts more: I would suggest a halterneck shape with more padding. A trick that some dancer friends I know use is the double bra - if you wear two padded bras, your breasts will look much bigger than they are
If you want to lift your breasts: An underwire bra with some padding is best.
If you want to keep your breasts low key (e.g. under a t-shirt): A less padded bra will be your best bet
STYLE
Here is where you have fun!
Don't let anyone tell you what you can and can't wear. Choose whatever colour you like and what whatever fabric / style makes you feel the most confident!
The only suggestions I would have here are:
If you're wearing white clothing, a brightly coloured bra will likely show underneath
If you're wearing tight colothing, a bra with a fun texture will likely show underneath and look a little funky