Wow, pretty good for a first try
Is this one screenshot? Or did you make this out of 2 or more screenshots?
Something you can try is boosting the anti-aliasing. This will cut down on the jaggies you see. Access your graphics card options and bump the anti-aliasing up. Also, there is an anti-aliasing option in the video settings on the AH main menu.
If your working with more than one image, try using layers in gimp. That way, if you mess up on one part of the picture, you can fix that one part without ruining the rest of the picture.