I have been photographing cars for a few years now. Biggest thing is experience, just keep going out and shooting. The most difficult part is choosing the location and time of the day. Mid-day sun will kill all the curves and make paint look flat and bright. Try shooting either in the morning or in the evening before the sun has come down. If only time available is mid-day, then try to find some shade. I personally love some of the shots I've taken at night. There you either need flash, light painting, or just use light thats already there (street lamp, windows, etc.)
Walk around the car and find details that make it stand out, if the car is long, you may want to try to capture that in your photograph. If the car is low, shoot from up high so it looks even lower. Just the opposite goes for a lifted truck.
Background should not be overbearing. You want the viewer to go straight to the car and not the interesting looking building behind it. There are exceptions to this: if you are trying to tell a story, for example a couple getting their bags out of a car at the hotel. You need to capture the whole scene so the viewer knows whats going on. Otherwise I try to either blur out the background or shoot against the wall or grass or bushes, anything that doesn't take away from the car. Another element to consider that I am guilty of doing all the time is having your background blend with the car. My car is gray, and at times I shoot it against a gray wall (you see the problem there?), luckily you can use PhotoShop or other editing software to change the color of the wall to make the car "pop".
Equipment...I've spent a lot of time shotting wide angle, pictures turned out great, but after a while that gets boring. Experiment with different focal lenghts. 3/4 shot of a long car at 17mm will definitely make the car seem even longer. I use Neutral Density filter on all my car photographs. This enables you to play around with reflections on car's paint and windows as well as the sky, water, etc.
Other than that, look through magazines to see what shots stand out, and keep on shooting.