Recently I felt like I wanted to buy a nice bra. May be a little something sexier than a sports bra which is what I've worn so far. I really don't need a bra for support as I'm only cup A.
I knew the basics of the sizing of the bra but the experience of buying one taught me a lot more than I expected so I thought I'd share my experience. I spent like 3 hours just bra shopping!
It's easy to buy a sports bra but with the lace style bra is totally different story. Yes, you can measure your breast circumference and figure out your cup size but it doesn't really end there.
Every bra is a little different and so will be the fit. I measured my circumference to be 40" and breast cup size A. Well, good luck finding size 40A! There are hardly any 38A. 36A is where you'll start seeing some choice. You must use strep extenders but that messes up the fitting of the shoulder streps causing them to fall off. The only solution is if a bra has detachable shoulder streps so you can get shoulder strep extenders. JOENA was a place I find some. They were already pre made so all I needed to do is mount them on. The bra strep extender will give you up to 4"of extension. Now you must cross the shoulders streps on your back in order to make this useful or you could try to get a simple shoulder strep tightener that comes in between the shoulder streps on you back but I personally went with the crossed shoulder streps in the back. Please note there are shoulder strep extenders and bra strep extender. Not the same thing
However if the space between the cups is not extendable you may run into cups being too close to each other which makes inappropriate fit and bad look. Smaller breast circumference bra implies your boobs being rather closer to each other which is likely not the case for the person of larger chest.
Also the bra extenders at Victoria Secret did not line up with the bra hooks. They were much wider on the extenders. The same thing at the Nordstrom. At JC Panny they were matching and they come as single, double or triple rowed hooks. You must match them up according to the bra of your choice.
Sales personal at Nordstrom works on commission which means they will be dancing around you and help you find the right fit. This means they will look at you in the fitting room with only the bra on you.
I found this very unnerving but you have to go along with it as it is just normal "Certified Fit Consultants" behavior.
Wax or shave your chest for that reason if you have some hairs and may be I'm paranoid but I'd tuck as well! I don't go anywhere without tucking anymore as it makes me feel very uncomfortable otherwise.
Padding is another choice to make. I was presented with various sizes of padding in cloth or silicon options at Victorias Secret. I have to say I was shocked when I saw how much cheating your money can buy.

I personally didn't get any but I went with padded bra. There are bras where you can remove this inserts out of their pockets which is what I would recommend. Mine does not have that option and although I went with minimal padding it feels like I have huge boobs. It doesn't look necessarily like that in the mirror but it is definitely something to get used to for me. Besides growing breasts tend to hurt so a little padding gives you a little protection too in case you bump into things which is what I do due to the type of my work.
It's FUN wearing a sexy bra, it makes me enjoy myself a little more!
'hope this is helpful, I wished I knew these things before I went in.