Tanya is giving you a good starting point to help you figure out what kind of sizes might work, but the only way to find a well-fitting bra is to actually try on a bunch of different styles and designs from different companies. If you think about it, it makes sense that a 30D bra made of stretchy lace with a narrow band and a single hook closure will feel less constricting on your body than the same size with a four-hook, wider band bra made of a sturdier cotton fabric. Band and cup sizing is only one of several variables you need to consider when looking for a well-fitting, comfortable bra.
And don't forget to check your shape by wearing a tight-fitting t-shirt over your bra (even if you don't plan to wear something like that in public). Some bras can give a projected look, while others give a more rounded shape. I personally detest the profile of molded "T-shirt" bras, but lots of women love them.
I only wear underwire bras, but, to be comfortable, an underwire needs to fit properly; the width and shape of the underwire has to mimic your shape or the wires will create uncomfortable pressure points. The shaping in the cup is also important; some bras are shaped for women with high, rounded breasts while others work better for women with a shallower upper/fuller bottom shape. Some bras have a wider centre gore for widely spaced breasts, while others have a narrow gore for women with close-set breasts. Once you start to understand your own shape, you can identify which styles are most likely to work for you.
