How do you buy your first bra?

Okay so, i have been on hrt for about 4 months now. And since I'm a little bigger my chest is already pretty noticeable and they are very sensitive, so i think i should get a bra relatively soon.

A friend of mine told me to start with a sports bra and i know multiple other trans girls who also did the same. But now idk how exactly i wanna go for that. I struggle a lot of social anxiety so id prefer to not go into a big store. And even if i did, I'm pretty tall and as i said a lil big, so idk if anything would even fit.

What are other steps i can take? What do i need to measure and look for when i want to buy a bra. Are there any good online stores in Europe (i live in Germany).

And is there anything I'm forgetting?