First of all, you need to link to the stuff you used when you made this: cloud photo, scan of Mio etc. We can't offer a complete opinion unless we know where you started from, after all!
As far as composition goes, it's a pretty basic wallpaper: not bad, but not exceptionally good either. I would advise you to clean up the image of the character more next time, since it looks like it's taken from a screenshot (blurry, odd colors). Best way to do it would be vectoring it, and it's not a difficult image to work on so following this tutorial would be just the thing to make it look better.
Good luck on your future works! :)