Let's start with the bra band size.
Now,take a measuring tape and wrap it around just under your bust as in the picture (1) and find your band size here:
How should your cup fit..
If you don't know your cup size:
Bust size(nr.2,picture above) - band size = cup size
If the difference in inches is 1 you are A, 2 -- B ,3 -- C and so on.
The underwire should lie flat against your skin (1) and your chest (3). It shouldn't be pushing out the front. It needs to be sitting behind the breast as well (2). You don't want your skin spilling over the sides of your bra. (4) You should go up a cup size if your bra is cutting your breast or down if that area is not supported and the bra feels loose.
First,you should know that not all A , B or C cups are the same. 32A is not the same as 34A .The 32 A cup is smaller. If your bra isn't fitting for any reason, for instance your band is lose ,you shouldn't forget to change the cup size if you're going to change the band. For every band size you go down/up you need to go up/down a cup size.If you are a 34A and need to go down 1 band size you'll take a 32B ,36B to a 32D (going down 2 band sizes and up 2 cup sizes in the same style of bra, don't forget this.)
You should experiment with different bra types to find out which ones work best for you.For example : A woman that wears a small cup may find it hard to find a fitting full cup bra and a triangle bra would work better depending on her shape.
If you have any specific question you can ask me in the comments :)