I found an old tutorial on a random pic I created once... :) thought it would be nice to share it with you..
Okay, here we go:
You got a sketch/drawing to start with.
the first you do is to create a new layer above the
one background (ORIGINAL) layer you've got.

There you start with the outlines.
When you've finished this the best you
do is save that layer with the lock symbol
otherwise you might draw in that layer
by accident.
You can create a new white/coloured layer as BACKGROUND meanwhile,
to check how the outlines will look in the end.
(This also helps with colouring..You can't see every colour
well on a white BACKGROUND, so I used a turquoise one to check
if I've overdrawn the lines and where I've to use the rubber tool
Check pics, I overdraw alot :P lol)
Okay, now when you've finished the outlines you
can create new layers for the colours.
I started with the layer for the skin.
Attention: Put the layer
under the outlines-layer,
so you won't overdraw your original sketch.

Now some details about the shadows and highlights with
the example of the hair:

Use the Burn / Dodge Tool to set shadows/highlights as you like :) + the Smudge Tool for nice effects



About the background, you can draw something using the paint tools
or just download some brushes and create an unique background.
If you're finished with the colour-layers you can merge the layers
by pressing ctrl+e :).
For the shadow in the background just use some layer effects (right click on a layer):

Result (big size on click):
