use this size chart to find out what size you are - small, medium or large. These measurements are in inches.
"drop" refers to the distance from the shoulder that you can use to determine the point at which you can measure your hip and waist. Allow for a tolerance of +/- 1/2 inch.
you might already know how to use a tape measure but here is a small guide to help,
just in case you're not totally sure.