Your waist is the bit in between your rib cage and your hips. It is often the most narrow point of your torso.
This measurement is one you will need all the time. To find it, simply wrap the tape measure around your waist, ensuring that the tape measure remains level all the way around (horizontal to the floor at both front and back).
Do not suck in your tummy like I accidentally did in this photo! You want to be as natural as possible, so that your clothes fit well. Make sure the tape isn't too loose, or isn't pulling you in either!