For one thing, I notice that you're using a 'Bucket Fill' method with an advanced image editor (eg. The GIMP, Photoshop, Corel, or anything not MS Paint.) to paint correct? I can see the effect created by this method by the whitish lines in between the outline of your character and the colour within. Well, there's the first thing you need to improve on. Instead of using just the 'Bucket Fill' tool, I suggest you use the 'select' tool, then, on whatever software you're using, increase the size of the selection by one px. This usually gets rid of the unwanted whitish parts, but it also has the nasty habit of mucking up your outline.
Besides that, from the way you paint your horizon, I can guess that you might at least be using GIMP. I could be dead wrong, but I'm assuming that for now. For one thing, you've painted the sky 'blue' with the paintbrush tool with 100% hardness, giving rise to those jagged edges over the horizon. I suggest using a softer brush or at least the airbrush tool to fix that.
Now, onto the lineart, I think that you're using a stroke path method or something along those lines (no pun intended). Otherwise, I'm not really sure how you got that jagged effect. well, if you are using paths, you should realise that you can 'curve' the paths right? As in it doesn't always have to be jagged edges.
Apart from the basics which I've covered, you might also want to work on shading too. I won't go too deep into that now, but at least try to add a shadow or something for next time. And that's shading in a nutshell. I'll elaborate further if you want.