Ohhhhhh you mean digitally! Yes okay, i was a bit confused about that, because it seemed like you knew very well how2pencil even a year ago. :v
In that case, you should probably check out the Flow parameter (often found close to Opacity), since that emulates pencil on paper more closely. It's more fidgety than a real pencil, but hey, might as well get used to digital and traditional handling differently (at least until you get some super fancy Cintiq or something). I don't usually use anything but 100% flow and opacity (i love me some big fat lines), but pressure-sensitive flow certainly seems like it's something you might be looking for. (and then you can vary the hardness of the brush and so on)

As for realistic shading, i'm afraid i ain't the best guy to talk about that, given my violent lack of experience in that particular area. :v
Also, most entry-level tips for paper also make themselves relevant for tablets (keep your arm mobile, follow the contours when shading, that kind of stuff), though i've personally noticed one big difference: On paper, you can get away with digging your pencil in and drawing a line relatively slowly. On a tablet, that's likely going to look unsteady as hell (depending on how rough the surface of it is and how good you are at keeping the pressure even), so you'll often get smoother lines with relatively fast strokes. Well, at least i do, can't speak for others. (post scriptum testing reveals that i might actually be wrong, good job me)