There is definitely an art to fitting a bra. Of course it needs to be the right size, but there a few other things that you need to think about too. You need to consider the style of the bra, the cut, the fabric and whether it is appropriate to you and your needs. Learning which bras suit you best comes with experience and much trial and error, however there are a few basics that you need to know about fitting a bra before you can even start to think about those things.
Firstly, the 10 (32), 12 (34), 14 (36) etc measurements refer to your frame size- not to be confused with your shoulder size. (contrary to popular belief, your shoulder size is not your frame size). The bra goes around and is secured in the narrowest part of your back. Think about it in terms of what size clothes you wear, this can help. If you wear size 10 clothes, why would you wear a size 12 or 14 bra. That just makes no sense!
The A, B, C, D, DD is the measurement of the cup. Cup sizes range from A or AA right through to an M in some brands, but most commonly to a G or a J. They go in order as follows; AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J. Only some of the sizes have the half size inbetween, like the D, DD cup sizes, this is because they are the most common sizes that need the half size in between. Most of the fittings I do are DD's, so lucky they do a half size in that cup!
When fitting a bra, it should fit you nice and firm around the back when secured on the loosest hook. This means that when the bra gives and stretches with wear, you have (usually) two rows of hooks to move in on the back and therefore will have the bra for longer.
The wires should never sit on your bust. It should sit comfortably on the sides of your bust and contain all of your bust within it. The front middle of the bra should be sitting right against your skin and be giving you a nice rounded, lifted but separated bust. There is nothing more unnatural and unattractive then breasts have been pushed in and up together. One word; chafing.
There is more that goes into fitting a bra and these are the basics of the basics, but hopefully it gives you a better idea of what you need to be doing if there is no trained bra fitter at your disposal!