I did say to make the lower part of the arm longer, but I think the arm in total is too... extended, in a sense. You might want to try and make the elbow more protruding and not have a curvy, rubbery arm.
As for shading, make sure not to use black outlines, unless you are outlining a dark shaded area. Otherwise, use a darker version of the colour you are outlining to, well, outline. Sometimes though, you may want to use black regardless (or close to black) to emphasize a boundary, which is commonly seen in Reborn's character sprites. But in general the colouration takes more practice and effort than skill that can be taught.