Ana, pencil works fine for hair. I find it helpful to think of hair in two ways. First, look for significant places where different tufts of hair overlap eachother. They are important towards building up the shape of the hair. Second, be aware of relative values within the hair caused by shadows and lighting. If you get the shading right and the general shape using specific, important tufts of hair as a guideline, the hair usually shapes up nicely. Hope this is helpful.
Artist Reply: thanks a lot Ari!! i'll try to do that. i started doing the top coz the directions of the hair are visible. i guess the lighting they used for O.B's pic is situated more on his right side.