Pretty decent for your first attempt
Don't be too hard on yourself.
I think the girl you used looks nice, well posed, is looking at the camera and she connects to the floor (rather than floating or sinking through it.) Also you avoided placing the camera directly in front of her which usually looks less nice.
To improve your image the most noticeable thing is the pillar in the foreground that pulls away your eye from the girl. I'd move it off to the side so it isn't as prominent in the frame. (like half of the beam visible at the edge of the frame)
I'd also zoom in the camera on the girl more. If you want a full body image I'd go for a vertical (portrait) image. If it is intended as a wallpaper you're stuck with the landscape mode but you might not need to show her entire body. maybe only showing the upper half of her body will allow you to fill more of the frame with the actual subject.
Last thing you could do for quick results is put in some dramatic lighting (blue, red, orange or green for the background plus a more natural light on the girl itself.
Hope this helps and good luck
