You do need a bar for it, but there are plenty of doorway pull up bars that are very cheap and can be fitted onto almost any door.
The pull up is a highly effective exercise that works the back, shoulder, core, and biceps. It is a high intensity exercise so you can not only build a great upper body with it, but also burn lots of calories and should definitely be part of your workout routine.
How to Perform a Pull Up
Grab the bar with an overhand grip (palms facing away from you). Your hands should be slightly over shoulder width apart. You can change the spacing of your hands from time to time as it does add variety to your workout.
Hang down from the bar with your arms fully extended. This is very important. A lot of people make the mistake of not extending their arms and this reduces the range of the exercise and its effectiveness. Stabilize yourself and cross your ankles.
Pull yourself up until your chin clears the bar. Do a symmetrical motion. Use your upper body only. Using your legs to propel yourself upward makes the exercise much much easier. This is actually called a kipping pull up and is done when you don't have enough upper body strength to perform the regular version of the exercise.
Lower yourself down slowly until your arms are fully extended again. Repeat the exercise as many times as you can for a complete set.
To build stronger and bigger muscles visit Triple Threat Muscle
For another great back exercise visit Supine Row Technique
John Davenport lost over 30 pounds in his twenties after being overweight most of his life. He now runs a weight loss forum and publishes a diet and fitness newsletter.