To start with, measure yourself -
Measure yourself using a tape measure and don't pull too tight. Ask someone to do it for you so it's more accurate.
Swimwear Size Guide
For bikini tops and 1 pieces:
1. Measure the chest horizontally at the fullest part of the bust.
2. Fallen for a 1 piece?
Measure your waist too: the measuring tape should be level with your belly button.
3. Measure the fullest part of your hips.