If the lines have been done in Pencil it can be a bit difficult so I usually do the lines with the pen tool on a different layer so they're nice and neat. But either way, how I do it is by using the magic wand.
Select the areas within the lines with the wand by clicking and holding Shift down so that next to the wand is a little + sign. It means it will add whatever bit you click on to the selection.
Alternatively if you hold down Alt you get a - sign.
Anyways! Once you have selected all the negative space around the lines go to SELECT and scroll down to inverse and you lines should then be selected! if you want to refine this selection go to SELECT again then to MODIFY and you will have options to BORDER, SMOOTH, EXPAND, CONTRACT, FEATHER. I recommend using the contract one and only by one or two pixels.
If you're after colouring things I generally just select the area with the wand expand the selected area by 1 or 2 pixels then colour the area on a separate layer which I then put on multiply. I hope this helps!