well the first thing wrong I noticed was that John Terry was in it...just kidding man it's not bad for a first effort. I have gimp, and even though it isn't as good as PS, it has many of the same features. What I would suggest is find a better background picture, because that one has some weird light effect which doesn't go with your rendered photo of Terry. Second, you need to work on the render, (your renders will be better the more you practice it). Those jagged edges can be fixed with the blurring tool or eraser tool, just zoom in on the photo and try to smooth the render out. Third, you should actually try to use some brushes in your background, it adds a bit of flair. Go to www.deviantart.com
and you should be able to search for photoshop brushes there. They have LOADS of gimp and PS brushes free to download. Go to www.dafont.com
for a very cool selection of free fonts. I get all the ones I use from there. Those are some basic things that would make the sig a bit better. Search google for guides on how to make photoshop sigs; there are loads of guides that can help you on just about everything.
P.S.- You can try to blur the outside of the render by putting the first layer on a gaussian blur, then duplicating it and pasting the original non-blurred render back on top. It adds a "glow" around the render. All in all though, not bad for a beginner. Hope that helped.