I'm not a pro but well, can give you those:
work more with the lines, don't let white spots
don't forget to color the background
work more on the shading, we can clearly see the brush on his face
Also missing is you enlarge the image size is not very girl guide you through the image of manga is always better enlarge them!
These learning ^ ^
like me!
mmm... so so... i'm not a great artist... but practice your lines! and if u don't know how to do the BG just paint it on white, expand the layer, and blur it! it'll make that the work bee cooler... but don't worry! i've just made two works! nice dude