Pull-ups are an old staple of any workout regimen and for good reason -- they're effective and can be done with limited space and equipment. All you need is a secure bar that is hung at least 6 inches (15 centimeters) above your head. Your local gym probably has a pull-up machine that will assist you by providing some lift. Pull-up machines are great if you're just starting out.
To get the most out of a pull-up, focus on getting the correct grip. It will ensure that your back muscles are fully engaged. Stand under the bar, and grip the bar with your palms facing away from you. Your hand position should be slightly wider than shoulder width. Bend your knees and let your body hang below the bar. Pull yourself up so that your chin clears the bar, then slowly ease yourself back down. Don't drop back to the hanging position -- it's inefficient and it could strain your shoulder and arm joints.
Pull-ups work your shoulder and upper back muscles (delts and traps.) Start with two or three sets of 5-10 repetitions. If you're new to pull-ups, you may have difficulty doing that much (or any complete pull-up, for that matter) but just keep trying and in a couple of weeks you'll see marked improvement [source: Murphy].