You should practice your drawing skills on pencil and paper before jumping onto Pixel Art.
Check your goal's lists:
1º) Improve shape and shading of the muscles. -----> Again, paper drawing will really help you. Check out some of the free online books in this
link. Especially if you're just starting, look for the ones of Andrew Loomis (
Fun with a Pencil)
2º) Learn how to shade a face -----> You don't need to learn how to shade a face. If you know how to do shadows in still life, you just need to use that knowledge and understand that a face is just another shape with it own particular planes.
Good luck!