To choose your size, you will need 2 measurements: bust and underbust.
Bust: measure around the fullest part of your bust. Often this will run the tape right across your nipples.
Underbust: around your ribcage, just underneath your breasts.
Taking measurements accurately is important (don’t rely on your ready-to-wear size). The cup letter may not match the one you usually wear, especially as there are many size systems depending on countries.
It is important to make a toile/muslin first, especially for cups above C as body shapes may vary. You may need to try another cup or size if the fit is not right for you.