Study the film and images from the film to get a good idea of how the character moves and looks. Once you're comfortable, pick a still or image you like and begin sketching Tarzan. Break it down into simple geometric shapes -- a tapering triangle for his chest, rectangles for his arms and legs. Concentrate on getting the posture and scale right at this stage rather than detail.
Once this is done, begin rounding the edges of your shapes to connect them together. Do this by sketching over the lines more than once as this will soften the lines and angles and give you a good idea of how the character fits together.
Examine the sketch and look at the angles and places where detail is required. Round the angles down to make Tarzan's body look more natural and sketch in facial features and the overall shape of his hair.
Begin shading to add detail and definition to the character. Look at how his muscles are shown in the film, and how his feet are a little more splayed than normal, as a result of being raised by gorillas. Pay attention to these details, and don't be afraid to rub anything out you're not happy with.
Focus on Tarzan's head, drawing a simple crosshair of two lines over the center of his face. Sketch his facial features on, over these lines, to make sure they line up.