A pull up is performed with your palms facing away from the face (reverse grip). Hang from the bar with your hands just wider than shoulder width, with straight arms and crossed feet.
Pull yourself up by imagining pulling down into your elbows or by imagining pulling the bar towards your chin rather than pulling yourself up. Your chin must go above the bar to be counted as a successful rep.
On the way down lower yourself slowly and deliberately and make sure at the end of the rep your arms are straight and you are really hanging like a jungle monkey.
To get the movement going grip tightly and pull the elbows down. When you max out, press stop, but then jump up and slowly try and add a couple reps of very slow negative declines. If you find pull ups too difficult at this point, then most gyms have big elastic bands that help support some of your weight. If not start by doing lat pull downs.