For the most accurate measurements:

Please ask someone else to measure you.
Do not wear bulky clothing, either undergarments or a leotard would be best.
You should be able to fit one finger within the measuring tape whilst measuring. Don't pull too tight!
Youtube has tutorials on taking measurements if you need help.
If in doubt, ask a local professional to get your measurements.

Bust - Measure the bust at the fullest part. Measure all around the body (total circumference).

Waist - Measure the waist where the body bends. It helps to bend side to side to identify exactly where to measure. You can put an elastic band around the waist to mark the correct placement.

Hip - Measure the hips at the fullest part, usually around the seat.

Sleeve Length - With the elbow bent, measure from the top of the arm (find the bone at the shoulder/top of the arm) to the wrist (find the bone at the side of the wrist).

Neck - Measure about an inch above your collar.

Inseam - Measure the inseam along the inner thigh and calf, from just below the crotch to the ankle.

Outseam - Measure along the outer thigh and calf, from the waist to the ankle.

Dress