Step on the scale to weigh yourself. Record this measurement.
Hold your bike along the top tube (the part of the frame that runs from the handlebars to the seat). Keep the bike as centered as possible as you weigh yourself with the bike. Record this measurement.
Subtract the weight in Step 1 from the weight in Step 2. This is the weight of the bike. This number is important because kickstands are designed to hold a certain amount of weight. Write down this weight and keep it with you when you are shopping for your kickstand.
Measure your bike frame if you don't already know its size. The frame size is the length of the seat tube from the seat post to the center of the bottom bracket. The bottom bracket is the point where your pedals attach to the bike. Knowing your frame size narrows down your selection of kickstands, since some kickstands are designed for larger frames and some are meant for smaller frames.
Hold the bike on a hard surface so it leans slightly to the left. Measure from the center of the bottom bracket down to the floor. This measurement (in millimeters) is the shortest kickstand you should buy. Have a friend help you with this step.