This is pretty good for someone who is not used to digital art - my lines looked all wobbly and crooked :) It's too bad you couldn't finish the picture on time, I'd have loved to see the end result.
Just keep practicing drawing lines with your tablet. I've been using mine for years and I still wish I could afford a Cintiq, it's just so much easier to get those perfect lines when you see your hand while drawing. But what I do for practice is look at Jazza's (or other artsits) tutorials and fast paint videos and draw something during or afterwards. They have great tips on working with digital art and different techniques. And keep it up :)