The absolute best starting point is taking your underbust=band size. It might not work for everyone BUT it's going to give you the best starting point (being that some women need to go up a band size and some need to go down). What you need to learn are the signs of a well-fitting bra:
- the band needs to be tight. 80% of the support from the bra comes from the band. It should be parallel to the ground and you shouldn't be able to comfortably put more than about 2 fingers underneath your band.
- The center gore should sit flat against against your sternum.
- the cups should enclose all the breast tissue i.e. no quadraboob.
- when you put on the bra make sure to bend over and scoup all your tissue into your cups. This CAN affect your bra size so don't forget to do this while trying it on!!! Additionally, if you're experiencing back/underarm fat then this can help prevent that.
As for figuring out your cup size it takes a lot of trial and error but I'd actually recommend a few different methods of measuring your bust because if you're in a really ill-fitting bra than your bust size might not be the appropriate one (due to your breasts sitting too low).
I give a detailed description here on how to measure your bust size and have a link to a great bra size calculator.
If you want to see the difference a well-fitting bra can make check out
this link. There are a lot of reasons to have a well-fitting bra (yes, health reasons)
that I outline here.
If you have any further questions or need help with fitting/where to find bras, let me know!