Many bra shops will have fitters at the store.
You can do it yourself if you want using a calculator like http://www.berlei.com.au/bra-size-calculator/
Or using a table like: http://www.brasnthings.com/Fitting-Room/The-Perfect-Fit/How-to-measure.html, especially if you have an unusual size, or are embarrassed about going to a fitter.
Of course, even once you know your size you need to try on each bra and check its fit. Look for no 'double boobs', a centre panel that sits flat against your body, and a back that doesn't lift up.