1. Above the bust: only for strapless dresses. Measure the circumference around wherever you want the dress to sit at the top.
2. Bust / Chest: for shirts or dresses. Measure the circumference around the widest part of your chest.
3. Waist: for dresses and shirts. THIS IS YOUR NATURAL WAIST, not where you would normally wear your pants. Measure the circumference of the smallest part in between your bust and hips.
4. Hips: for pants, skirts, and dresses. Measure the circumference of the widest part of your hips. Pretty much, measure around your butt. I guess "hips" is PC.
For skirts or pants where you want to wear them below your natural waist(and trust me, you do): Measure the circumference around wherever it is your pants or skirts normally sit at the top. Be sure to note that measurement carefully or I will be confused about what's your natural waist and what's the place you want to wear your pants.