Pull Ups / Chin Ups Arm Exercise
There's loads of ways to do this exercise, I have shown the easiest way for those of you who are just starting out. After you get better at them then you really want to change the grip to the one shown, put your hands wider apart with palms facing away from you - an excellent exercise for your lats, biceps and forearm. Do the full range of motion, go down until your arms are nearly straight and come up until your chin is touching the bar, do not touch the floor in between reps as this makes it easier...
If you have problems doing this exercise at first then I would advise you get a partner to help you (or use a chair to help you up) then lower your weight by yourself (ie. negative reps) or try doing half reps (ie. doing only the top half on the exercise and as you become better lower yourself more).