Measuring for your bra size depends on the measurement system you are familiar with.
If you live in the United Kingdom or the United States you will be more familiar with an imperial measuring system
(e.g. bra sizes like 36D). If you live in Europe or Japan you will be used to metric measuring (e.g. bra sizes like 80D).
However different countries use different sizing methods, for example France, Italy or Australia.
Please also be aware that not every brand fits the same and fit may vary with the bra model you choose.
Anita has a very in-depth guide to finding which of their bras will work for you. The first link takes you to their size calculator the others help you determine which style will work best for your body.