Pull-ups are performed using your own body weight however for more advanced lifters and once you become use to your body weight you can add weight to your body through the use of a belt and dumbbell or free weight .
To perform pull-ups grip the bar a little wider than shoulder width apart. Pull yourself up and make sure at the top of the movement your forearms are perpendicular from the ground. Try and make sure your forearms are not at an angle at the top of the movement. If this is the case then it means your grip is either too wide or too narrow. Squeeze your back at the top of the movement and try and have your chest reach as close as possible to the bar at the top of the movement. Control the downward movement and do not just drop your body weight, instead move down in a slow controlled manner. Repeat the process.