YukiHere's some stuff that I've typed up for others, it gives tutorials on shading and some extra tips:

Shading:
For as far as I can say with tutorials, I recommend Sneeze7's Where to Shade for when you're starting to begin the shading. After that move on to studying some other skinner's tutorials such as Leostereo's more advanced tutorials. I have uploaded some myself too, but I'm not here to advertise, I'm here to help out. Also while you're at it, check out the_soup's How Not to Shade Arms and Legs blog.

Another method I recommend using is finding skinners you like and studying the way they shade their skins. For example, I would look at where they placed their shadows and highlights on the skin and then attempt to recreate that myself on a new skin. While shading the skin I would alter the style bit by bit and merge it with other people's styles into something that I find to be a good result. If you are to upload any skin using someone else's style, I recommend asking them if it's ok first and then crediting them.

If you ever get stuck on shading or colouring skins, feel free to PM me as I would be happy to help out.