To solve your pixelation problem, the answer is simple.
DO NOT save as a jpeg!
The png format is your friend.
You said gimp doesn't allow you load your scanned works...
That shouldn't be possible. Try the "open as layers" option. This will open your work on a seperate layer.
Also, on the subject of layers, take some time studying the layer settings. Some of these allow you to color behind your line-art.
If you're not at that point yet, try to find out what layers are and how to use them.
Also, once you're really planning to get serious, get a drawing tablet. (a cheap one for starters.) If your parents need convincing, a tablet can be usefull for homework and stuff too.
manicjudgement
I shall not agree nor shall I disagree. This is the truly human way.