To measure your bra size, put a tape measure around your chest underneath your breasts. It's probably the same as your waist size plus a few inches. That tells you your band size.
The bigger measurement is at the widest point, around your nipples. The difference between the two measurements tells you your cup size. A 1-inch difference is an A cup, a 2-inch difference is a B cup, 3-inch difference is a C cup and so on.
My chest measures 42 inches around under my breasts and 45 inches around them, so I'm a 42C.