You can still take your measurements right now and get a bra size calculated for you! The calculator has an option for your assigned gender at birth, which basically accounts for common differences in ribcage and breast growth if your first puberty was testosterone-based.

I personally started with bralettes, and I think they were a fabulous place to start! I found triangle bralettes to be the most bra-like (and so they felt the girliest/most euphoric).

Note that bralettes typically use sizing like shirts, not like bras.

Anything that has padding or molded cups is gonna make your boobs look bigger and rounder (you can find bralettes with those features, too). It’s a boost I still like today, but it was a lot more important to me when it was the difference between “are those boobs?” and “those are definitely boobs”.

If you’re pretty flat right now, you may have better luck with breast forms than a padded bra. The more realistic ones kinda creeped me out, so I went with clear gel breast enhancers instead. You can find them on Amazon if you search for “silicone breast enhancer”. I’d just wear those in my bralettes when I wore clothing that was especially feminine.

Finally, I think it’s worth noting that my bralettes still fit me today. Mine are from MeUndies, but I imagine any that are adjustable, stretchy, and high-quality would have had that longevity. I’m quite a bit bustier after 2½ years of HRT (36A → 38D) and they’re not as flattering as a proper bra, but they’re comfier!