The more measurements I have the better the fit will be, the measuring tape should be snug and flat not pulled too tight or left too loose for these measurements.
The basic measurements are Bust, Waist, and Hips. Bust at the largest point typically under the arms around the fullest part of the breasts, Waist (natural waist) at the smallest point typically near the belly button or the middle of your hourglass, and hips around the fullest point typically near the bottom.
For more in-depth measurements include under bust, tight waist, rise and back length.
Under bust is under the bust around your rib-cage right under your breasts, typically how you would be measured for a bra. Tight waist determines how squeezable you are, measure in the same place as a standard waist measurement but with the tape pulled as tight as you can stand it. Rise is your natural waist/belly button to lower hip or groin area, and back length is the base of your neck to your natural waist. These last two are especially helpful if you feel you have a long or short waist.