Ok what you need to do is...
Firstly measure your back - say this is 30 inches. You then need to add 4 inches to this, to get your back size - so it would be 34 inches. (If it was 31 inches, an odd number, you add 5 inches, so it would be 36 inches.)
Then you measure round the fullest part of your bust to get your bust size. Say this is 40 inches. You then need to work out the difference in inches between your back size and bra size, and this gives you your cup size.
EG.
A - 0 inch difference
B - 1 inches
C - 2 inches
D - 3 inches
DD - 4 inches
E - 5 inches
F - 6 inches
G - 7 inches
Etc etc. So with a 34 inch back, and a 40 inch bust size, you would have a cup size of 34E.
That's just a guide that might give you an indication, but everyone is right you should get it done professionally. The women who do it see people EVERY day with their top off, they've seen fat people thin people people with breasts big, small, wonky, saggy, the lot! lol, they honestly won't care.
Wearing the correct bra size is important to support your breasts and stop premature sagging as well. They also recommend what sort of bras you should wear depending on the shape of your breasts.
British Claire, my friend wore a 36C for years, got it done professionally and they gave her a 32E lol!