DO NOT use the fill tool. It's the devil's curse. Use layers (on the left of the screen) Your lineart needs some work, it's too sketchy. Try using the air brush to make thick lines then come back with the erasure on med/low to shave down the lines. It'll be cleaner. There's more to fix but I'd be writing an entire itinerary. Keep working and improving, and do spend more than 16 minutes on your pieces. The results show. Go back and clean this up, it'll look so much better. Welcome.