definitely practice some more and focus on just this piece as well rather than making more than one. For the sky I would say opt for cooler lighting, too much warmth in one digipaint is an eyesore, for me anyways so throw in some cool colors. Don't worry too much about the horizon, focus more on the detail of that one mountain thingy you have, add in some rocks in the forground, and just have small jagged mountaintops dotting the horizon.