Strip down to just your underwear when you measure. You need these measurements as accurate as possible, so ditch your modesty and minimize the clothes.
Stand straight and wrap the tape measure around the fullest point of your bust. Slip your thumb between you and the tape measure -- this adds just a smidge of extra to the measurement for comfort. Note the measurement; this is your bust measurement.
Keep standing nice and tall. Wrap the tape measure around your natural waist. This is the smallest point of your waistline -- possibly your favorite part of your waist. Note the measurement and round it to the nearest half inch; this is your waist measurement.
Put your feet about 6 inches apart and stand up straight. Wrap the tape measure around your hips at the widest point -- right across your backside at the fullest point of your hips. Note the measurement; this is your hip measurement.
You'll need a girlfriend to help you measure height. Wear the heels you will be wearing with the dress and stand up straight. Have your friend place the tape measure at the hollow of your neck between your collarbones. Pull the tape measure straight down to the point you want the gown to end (the floor for a floor-length gown). Remember to pull the tape measure out away from your body if you think the skirt of the gown will be full. Note the measurement; this is your "hollow to hem" measurement.
Take these three measurements with you when you shop. You'll need them, especially when you shop for formal wear. Compare your measurements to the measurements of the dress, making sure that yours are the same or less than the dress measurements. You can always have a dress taken in, but letting it out is not so easy.
Your bust and waist measurements are the most important. Hip measurements may not be critical, depending on the dress style. If the skirt is full, your hip measurement won't matter at all.
Don't pull the tape measure too tight as you're measuring. It just needs to be snug -- if it's super tight your dress will be too.