Okay, so with the shading it's fine to start out in black only since it's best to keep things simple until you have the rough shape of the piece. If you want to go with the official style most closely you want to have almost all outer lines be black (or whatever the darkest color you're using is) and you want to keep from using too much of it within the piece itself. Save it for the areas that need to show the most contrast, like, for instance, if an arm is going across the chest you'd want to make sure that arm had some black outlines to make it stand out. Parts of the eyes probably should have some black in them, and anywhere else where you're dealing with dark colors and need something darker for the outline. In general you want to keep black outlines away from anything too light, so you wouldn't want some around the top of the hair except for the most outward outline, of course.
Facial expressions are pretty hard. I usually start by putting a vague line for the curve of the nose and something for the eyebrows or the tops outlines of the eyes so I have a simple idea of where the facial features will go, but not have them be so detailed that I can't move them around if I need to. I start things simple. I get the skin color and a darker shade of that and give it a quick coloring. Then I just a dark color, sometimes black until I can find a better color, and draw a quick and ugly shape for the mouth and the eyes. I'll add the whites for the eyes, and eventually some color for the irises. Then I'll pick a brown or whatever color and go over those black lines on the face and try to add more detail to the mouth and eyes. Somewhere along the line I'll stop and look at the piece and see if the positions look okay. Maybe the mouth is too far to one side so I move it, or the eyes seem too close together, or one seems too large, or something. Everything after that is just little bits of fine tuning.
Of course I also look at the official sprites to see what techniques they use. Maybe I'm having trouble with the eyes and I'll look and see something that could help on a similar sprite. Basically, my tip is to make something and then fine tune it. Save every stage you work at in case you make a big change and don't like it. Every now and then I'll be on my 30th stage and realize that I want to use something I made 10 steps behind so I can still go back and use it.