Alrighteh, first you scan a picture, or make a picture on the computer (please note that you have to work slightly different in the beginnen when you start making your picture on the computer without tracing the lines of another picture). I'll go with scanning now. Something like this:

Please realise that the image is not 100%

When you have scanned your picture you will have to create a "new raster layer". You can either name it or leave it as it is. Like this:

On this layer you will create the line art. When you are finished with the lineart, you move back to the background layer (which you can now conceal -- by clicking on the eye next to it) and create a "new raster layer" again.

Concealed layer (sorry it's so small):

When you have created the new layer, you have to colour it, but only with the primary colours you will use. I prefer colouring everything on a seperate layer (Layers like: "Skin", "Hair", "Eyes", "Clothes", etc.), but you have to figure out by yourself what you like best.
When you have coloured everything, you should create another raster layer to add shadow and another to add light. (I usually create the shadows first.) To make it all a little easier to see what belongs to what, you can also create so-called "Layer Groups". Like this:

That way you can see things clearly and so it's easier to use.
When you finished added light and shade, you merge all layers and then you're done 8D

I hope it helped a little... and good luck!!
(if there are any more questions, feel free to ask them ;D)