Getting a bra to fit is a very difficult task, particularly in large band and small cup sizes.
The band is the easier part, you measure your size under your boobs and that's it. Some bra suppliers insist that you then add a few inches, but I think this goes back to old cotton bras, which didn't stretch like modern fabrics do.
Basically,to give full boob support, the bra band has to be quite tight when worn low on your back and under your shoulder blades. Ideally it should sit below your underwire line. You should only be able to pull your band out about 4 fingers with new bra on the loosest clasp setting (the bra will stretch with washing).
Similarly, your bra straps should be set very firmly. You should only be able to pull each strap up to your ear lobe. The truth is that if you wear a bra all the time, you develop slightly grooved shoulders, and your straps then slip into these grooves and stay put!
A new bra should feel really snug & tight. This takes some getting used to if you are new to bra wearing. A bra which seems comfortable in the shop will go slack & saggy after a bit of washing.
The cup size is really difficult and you simply can't assume that 38A is equivalent to 36B. Having got your band size established, you simply have to try on different bras to see what works.
I have so-called AA cups which would allow me to hold up two sets of breasts, whilst I have had A cups which squashed my nipples flat.
Even a particular brand is no guarantee. I have bought M&S bras which give a 2:1 difference in cup volume for the same nominal size.
I have taken to bidding for pre-worn bras on eBay. You can get them quite cheaply, they are already broken-in and if they don't work, they can go to the charity shop.
You can also engage in risque conversations with the lady who has been wearing them!
Also bear in mind the fact that your bra may well be visible through your shirt or blouse. You need to present a nice snug, low, horizontal band, pretty vertical straps and a colour which compliments your top. Although a white bra is generally more visible, just don't wear a black bra under a sheer white top. If in doubt, wear a nude coloured bra. Even if it shows it denotes modesty.
Women do notice these things. Visible white is virginal, but black or red is tarty!