Not bad for a first try, but it could still be improved. Here's what I think:
1. First thing that really matters is composition. Best rule to follow here is the rule of thirds if you're not
centering your subject. Put guides throughout your canvas that divide it in thirds horizontally and vertically, then
position Takkun along the axes. The tops of the buildings should probably be aligned with the bottom horizontal third,
and you should put that tallest building on the opposite side of the pic, or take it out completely. It draws attention
away from Takkun. And I think it would look better if he were also a bit bigger.
2. Vector the buildings, the background, and Takkun. There's a lot of JPEG artifacts all over your wall, which
never makes for good digital art. Also, you can put some texture overlays. Be careful of going overboard with textures
and gradients, though. I see you're going for a slight minimalistic feel (which vectors can pull off really well),
and if you get it too busy you might kill the design. Take a look at some of the vector pieces in Desktop Anime. I think
there's a couple walls there with almost the same theme as yours, and you can lean on them for some
inspiration.
3. The rain's blurriness makes the whole thing look shoddy and haphazard. You should do something about it,
although I'm really not quite sure what. ^^;
4. The flat gray of the buildings contrasts a bit too painfully with the red sky. The sky is actually the only bit I
like (except for the artifacts), so maybe you should make the buildings a little darker, and a little on the browner
side, so the color just sort of sits in well with the background. Takkun looks fine as a solid black sillhouette,
though.
5. Take out that black bar at the bottom of your wall. Watermarks are called water marks because they're text right
on the art itself, so you shouldn't give it a background color of its own. But it shouldn't stand out too much
(people already hate watermarks as it is) so a white sans serif text with some thin stroking or a faint drop shadow
should be good enough.
Just some suggestions, as it's your wall and it's entirely up to you. If you don't wanna touch anything
else, at least fix the composition and the coloring. They're usually the first things that make or break a piece of
art.