There are inserts you can buy to fill them out if you need to. I don't know how far along you are with growth (from HRT), but I'd maybe recommend starting out with a bralette if you're just looking for something that fits whatever size and want a bit of lift and shape. I got a nice calvin klein bralette on Amazon when I first started.
I also bought my first actual bra from TheBraCompany, but found they were meant more for petite women (I'm 130lb, and 34/36 for my band size depending on cup. They were still crazy small!). My first comfy bra was a demi / tshirt bra from VS.

For proper fitting you can look up how on the internet. If you're planning on filling it with inserts maybe just get your band size and order an A to start (that way you can grow into it as well). You can also go to various stores for getting measurements taken. Nordstrom is amazing at this (but maybe buy from someplace cheaper, lol).
If you need links for anything, feel free to PM me. I don't know if we're allowed to post them..i just kind of came back after a long time of being away, so the rule on this may have changed.
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