Didn't really take that long. I just need advice for the bg. I have now idea what to do. Any suggestions?

Oke the background is not yet a worry, first try to get the scan good
it is fairly good extracted only like at his hair it needs some finetuning, its not very pretty looking over there.
The lines and stuff dont look bad on the wall but then please dont use a swirling pattern behind it in white. make that red ore sompt.. ore go for a completely different background and go look around for inspiration on tech wallpaper ore artist wallpaper sites.
besides that for now would not have a idea on whut to improve, try to fix those 2 first
good luck ^^

The background is little bit simple & empty~~~the brush work is not nice ~~~ Sorry~~~

A pretty good start. As Beckor mentioned about the hair, the extraction of the hair isn't all that great, there are white bits on Sasukes hair, though it's not all that obvious it count as bad extraction. Another bit is Sasukes chest, the edge has alot of white bits on it, and its also really obvious. Generally cleaning up the scan will be best ^^ Another thing about the scan, is that it's sort of grainy, maybe finding a better quality scan of Sasuke or another idea is to vector the scan so it's real good quality.

You need your sig on the wallpaper too, at the bottom of the wallpaper. I like the red bar thing which goes right through the wallpaper. But I think that this scan of Sasuke is best suited for a scenery background. Heres my idea for the background. You scrap this background and start making a cloudy sort of night sky with a shining moon at Sasuke. And maybe you could have Sasuke standing on a building? Like buildings in the old times, errmmm like in the episodes of Naruto, maybe you could screenshot one and vector it. And then generally adding more stuff to the scenery background so it looks more like a foggy night. Hope that helps you~! Good luck~! ^.~