To paint a car you need the template, this is just a flat view of each aspect of the car laid flat on an image, what ever you put on the template (ie the car) will apear on the 3D version of the car in game.
The templates are in a format, the format type is ".psd" this is a layed formate, and you need special software to open the files, the most used sofware is either Photoshop (the best) or Paint Shop Pro, if you don't have either of these installed on your PC when you download the file your PC won't recognise it.
If you can't paint a skin yourself someone else will do it for you, you can post a request for a skin on the skin painting section of this forum, click on this
There is also a very good forum thats dedicated to skins and other 3D Aspects, click onto the Skin Painting Forum
When you post a request make sure to say what you want, like what car, colour, what name and number, and anything else you want on the car. If you do that someone will get back to you shortly.
Rikard