Barbell Curl

1) Using a barbell, start with your hands shoulder width apart and completely extended.

2) Keep your elbows locked into position at your sides.

3) Without swinging your back, or using your legs, pull the bar toward your chest until you cannot go further.

4) At the top of the exercise, you can curl your hands in slightly to work out your forearms as well.

5) Lower your arms back toward your pelvis to end at the extended arm position.

Notes:
In order to isolate your biceps, keep your elbows locked at your sides and only move the forearm up and down.  Do not swing your back in order to gain momentum, this will not help you grow your biceps, lowering the weight and using good form with slower reps on the way down will achieve this just fine.  Keeping your legs bent is fine, but do not use them to help with the exercises, keep them firmly in place where you begin.