Maternity sizes work the same as regular clothing sizes. That is, if you're normally a size "small," you'll still be a small in maternity sizes. Same thing with numbered sizes: If you were an 8 pre-pregnancy, look for items in that size.
See the charts below for a quick guide to finding the size that fits you best!
Bust: With your arms relaxed at your sides, measure around the fullest part of your chest.
Hips: With your feet approximately 6' apart, measure around the fullest part if your hip. This area usually corresponds to the top of your legs.