well you need to decide which type of shading suits you first, I sugest trying soft shading and cel shading, and decide what you like doing the most then start messing with it.
1. the classic anime "cel-shading" where the light differences are made using solid color, skin tone doesn't very much. Its easier to do out of the 3 styles, but there is alot of room to mess up.
2. "soft-shading" This where more realism comes into the colors. The skin tone is very distinguished this way, and your light source is VERY important to keep and have flow throughout the image. This is my personal favorite way to do it, but it takes ALOT of time to do next to cel shading.
3. "Comic style shading" its kind of a mix of soft shading and cel shading, i personaly like the style alot and hope to beable to do it sometime.
The first two are made by the artist preciousflame which you can find her gallery on deviantart, the last one I don't remember but he has alot of steet fighter pieces. you can see his work in the street fighter comic.
left to right:Cel shade,Soft Shade,Comic Shade