Well, the canvas is way too big for the actual work.. SO get that fixed in the first place.
Secondly, make it look more interesting, throw some lighting in there or so as it doesn't look very appealing at this moment.
I'll give further CnC once you have completed these two steps
Image > crop > crop transparent does the trick nicely.
Agree it needs some better light, it seems fairly flat and bland. Try adding some effects in both the background and foreground, which will also help with depth. Subtle additions like glow, drop shadow and reflection could spruce up the work also.