bra size Watch
They measure you while you're wearing the bra you came in wearing and work from there. They leave the room while you change bras.
You leave the bra you currently have on and they take your measurements, give you a bra they think will fit and leave the room while you try it on. They'll come back in when you're ready and will see if it is the right fit or not. If not they change the bra and if it is the right fit, then great.
I went to Boux for my first bra at probably 12, after wearing sports bras for a while I decided I wanted an actual bra. I went with my mum but go with anyone you're comfortable with. They're professionals they know what they're doing. It's not too awkward, you may feel it at first but then again it's their job so no need to feel weird about it. I still go in for fittings now. Whether it's because I want to try a different bra type to the ones I have, or if it is to double check that I don't need to go up a size.