Sierra - Terrific job! I couldn't have put it better myself! ^^

Aniu - Sierra has given very detailed instructions for you. I'm going to work on some tutorials soon myself. (They will include screenshots.) I'll have to make this into a tutorial. If I do, I'll let you know! ^^

I'm assuming you want to make lines paint-friendly (pixelated) ok, so this is what you do.

Open your image you want to make a lineart out of.

The bar on top of your screen has a bar across it saying file, edit, select, exec. Click on layer, then click on new layer (it should be the first one)

Make sure your new layer is a transparency.

Then click on the pencil tool in your toolbox and select Circle (01) as your brush.

I would also suggest changing the foreground color to something other than black so you can see your lines (I use bright green)

Now find the path tool on your toolbox (if you have my version of GIMP it should be next to the color-picker tool on the left)

Now click on a spot along the area you want to outline. You should see a circle there marking where you clicked. Then click on another spot along the area you wish to trace and a line should form. You can bend the lines so they match your image as close as possible.

When you've got your line (or path) where you want it. Click on Edit on the same place where you clicked Layer. You will find a Stroke Path option near the bottom.

Click on stroke path and a window should pop up near the bottom is an option that says stroke with a paint tool. Click on it and find your pencil tool than click stroke.

You should have a new line along your path. Keep making paths and stroking them along the lines of your original image. I would suggest doing things in bits and pieces and TAKE YOUR TIME. Rushing things makes them look ugly.

After you've traced the entire image, hold the ctrl button (on your keyboard) and hit the L key on your keyboard. Your list of layers should pop up. You'll see your new layer with the lines on it and your original image.

If I'm just making a lineart I'll click on the original image from the layers window and hit the Edit option again and fill it with the background color (white). So all you see are the bright green lines.

In order to make the lines black again find your select by color tool in the upper right hand corner and select the green (or other color) in your lines. Then click on Edit and find fill with foreground color (change your color palette to black and white not green and white) The lines should turn black like you wanted them to.

Then hit the Layer button again (where file and edit are) and click merge down. Now you have a lineart with just one layer.

I hope this helped and wasn't too confusing.