Bust: Measure around the back, under the arms, and across the bust at the fullest part. It is best to do this with a bra on (not a pushup bra). 

Waist: Measure around the smallest part of your waist. 

For comfiest fit, try to not suck in or pull the measuring tape tightly. 

Tip: If you do not have a soft measuring tape, use a piece of string and a ruler.