To make things look clean, I'd reccomend using the smoothing tool (this is necessary for me to draw) and set the spacing to 0. I'd also use the pencil tool if I were you. ^^
You can easily shade by making a new layer (preferably above the colour but under the lines) and selecting from the dropdown list 'Mutliply'.
After that, just shade and you'll see the effects. To make it lighter, rather than choosing a lighter shade, you should adjust the opacity of the layer.
To add highlights, select 'Add' instead. ^^ (to demonstrate this, I added a splash of colour)
If you're feeling lazy, you can use the burn and dodge tools on the colour. Burn makes it darker, dodge makes it lighter.
I am assuming you know about the other brushes?
