Raise your arms slightly and measure around your body. Be sure to measure the fullest part of your chest/bust, keeping the tape parallel to the ground.
Measure around your waist where your pants usually fall. Keep one finger between the tape and your body to ensure an accurate fit.
Place your hand on your hip with your arm bent at a 90 degree angle. Measure from the middle of the back of your neck, across your shoulder and down your arm to your wrist.
Find a t-shirt that fits you. Measure from the center of the back collar of your favorite t-shirt to the end of the shirt, this is your desired length.