There are lots of ways to do this but here's my suggestion:
* Lock the layer that contains your sketch so that you won't edit the lines by mistake while adding colour.
* Create a new Layer and name it "Paint".
* Move it behind the layer that contains your sketch lines.
* Go to the Shape Painter tool, choose a colour and start to "colour in" the shapes.

You can use the Eraser tool to get rid of any mistakes you make (and undo of course).

Once you've got the basic shapes, you can edit them in all sorts of ways, adding more detailed fills, giving them soft edges, etc., etc...

Phil