Finding your bra size for a particular make is a protracted process!
It doesn't help that there are different measurement methods.
The modern way is to measure under your boobs, and that is your band size. Then. preferably wearing an unpadded bra, you measure over your nipples. If you are 1" bigger, it's a A cup, 2" bigger, it's a B cup etc.
But all the bra manufacturers seem to have slightly different standards, and it also depends on the style you desire: plunge, balconette, unpadded etc.
I resorted to buying cheap pre-owned bras on eBay, so that I could try the various combinatons of band and cup size. Those that didn't fit, I sent to the clothes bank.
I would add that if you are not used to wearing a bra, then an intitial wearing might appear to be impossibly tight. A bra is a support garment, and it requires a tight back band and shoulder straps.
It will mark you!