Well it's not very good..
Since you're a beginner, you best follow some tutorials. There lots of them on this site.
Try not to do so much at the same time. Start with just lowering and changing rims.
If you got that covered expand the changes you make on each car, change background, copy/paste some bumpers, change color and so on.
Are you working with photoshop? If not try to find a photoshop version and use that.
Greetz Kevin