Taking accurate body measurements is the single most important step in finding clothes that fit well online. Here's a comprehensive guide to getting it right.
You'll need a soft, flexible measuring tape (the kind used for sewing, not a rigid carpenter's tape). If you don't have one, you can use a string and then measure it against a ruler.
For chest measurements, wrap the tape around the fullest part of your chest, keeping it horizontal and parallel to the floor. Don't pull it tight—you should be able to fit a finger under the tape.
For waist measurements, find your natural waistline (the narrowest part of your torso, usually above your belly button) and measure around it. Stand naturally—don't suck in your stomach.
For hip measurements, stand with your feet together and measure around the widest part of your hips and buttocks.
For inseam, measure from the crotch seam of a well-fitting pair of pants down to where you want the hem to fall.
Pro tip: measure yourself wearing the undergarments you'd typically wear with the clothing you're shopping for, and have someone else help if possible for more accurate results.