There are many tutorials for shading skins, even though there don't seem to be many for clothes.
I usually make a palette full of light to dark colors, then try to shade the clothes in a realistic way.
Think about where the shadows on a sweater would be, probably on the insides of the arms, legs, etc.
It can also help to think about the texture of your sweater, so you can make sure your shading fits with it.
Make sure to know where your light source is coming from, in order to decide which areas need to be darker/lighter. :)

Sorry if this didn't help much, I tried my best to explain, but I'm not the best at shading in general, nevermind clothes.