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'.
![Image](http://i62.tinypic.com/2ll14b7.png)
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?