just to be sure you know, the best way to make a roller coaster is by combining a bezier curve or path with a bevel object to make the track, and then parenting the car to the path for animation. if you just want to give basic materials to your cart, check the blenderwiki chapter on materials. If you want to get complicated and painterly, you have to learn UV mapping, and unwrapping. As for the background, that is complicated, because your point of view will be constantly changing, so a simple image file will not do. you’ll need to build some kind of basic ground and ground objects, and for the sky, you can use blenders texture channels for the world functions, or put everything inside a big sphere, or ‘skydome’. you can also use the cloud generator to give it some real depth.
(edit) the discombobulator script might be helpful with faking some buildings.