To perform a chin-up, hold onto the bar, use the arms to pull-up or chin-up until the chin is level with or above the bar. Lower again and repeat according to your capability. Pull-ups / Chin-ups can be done with two hand grips, forward or reverse, each strengthening different muscles. The forward bar grip (backs of hands facing you) places more emphasis on the trapezius and muscles of the back, whereas the reverse bar grip (or underhand grip) is narrower and places focus on the biceps. The chin-up bars can also host many more exercises, including shoulder and arm stretching, improving the grip, the hanging leg lift which strengthens the abdominal muscles. In addition, simply hanging by the arms to stretch the lower back and align the spine is a great benefit to the lower back.
Introduce yourself to all exercises gradually, avoiding undue stress. Slowly build the number of repetitions. Most people find chin-ups very difficult, especially those with higher body weight. As necessary, use the ground to help boost upward motion.