Comment on Sibling Rivalry - Reloaded by Zunkuft
Looks good for a first attempt; using Gimp just takes some getting used to. I'm guessing you scanned it and used the paintbrush to color it all?
It was a pencil sketch already scanned, I just messed with the Threshold and contrast tools until I got a nice lineart out of it. Then I went to work with layers of different colours 
Messing around with stuff is often the best way to learn
I hope you continue to make progress!