Ah, sorry for the delayed answer, real life is keeping me anyway from everything.
About layers, I think the easiest way to learn about them is just to play with it. Like, make yourself a simple sketch or something, and then try the different options and check the various boxes around until you understand what it does. There's no real tutos or explanations about those, or they don't really help much...
About the lineart thing, I suggest you try the simplest thing: color to alpha! Select the layer your lineart is on, the go to Layer-> Transparency -> Color to Alpha. Select the color of your background (usually white), then click Ok, and Voilà! The background is gone. Then, you can just check the little box in the layer panel to lock transparency, so that when you color your line, if you choose to do it, the background will stay untouched. And then, you can just use any painting tool to change the color of your lines.
The method showed in the tuto you linked is good too, though a little different. Since I usually don't ink my picture and work with cleaned sketch, I prefer the color-to-alpha thing 'cause then you don't need to select anything, which can be tricky if your picture is pure B&W. Though, if you want to try it, it's just a matter of selecting the white on your picture (I suggest using the "Select by Color" tool), then inverting it (Select -> Invert), so that your active selection is now what's black on your picture (your lines). Then you make a new, transparent layer and, with your selection still active, you fill it with the bucket tool. Then just click the little eye in the layer menu to hide the original picture, and the "lock transparency" thing on your new lineart layer. You should now have a clean lineart on a transparent background, and you will be able to re-color it using any painting tool without changing the transparent zone.
Hope it was clear and helped...