I can see that your using Sketchup. Nothing is wrong with it, however, it will be alot of work moving the files from Sketchup to Zmodeler. As Freelancer said, try use 3DS Max. Just remember you won't learn it overnight, and making car models can take a while. However if you take your time and watch Youtube video's I guarantee that you will make models that people will find better looking. You can use other programs if you wish aswell (Blender - fully free, Zmodeler, Maya, etc) For reskins, I'm not sure what program you use but I suggest Photoshop, or if you can't afford it GIMP. Try not to use the paint bucket (but you can for some parts). Use paintbrush and pencil tools for smaller details. Ofcourse there are tutorials for this aswell. I like the general looks of the models you made though. Keep the concepts and as you expand you 3D knowledge create smaller details that will make it look nice