I'm still very much a noob with "proper" photography but I'll share my very limited knowledge on the subject :p Remember that long shutter openings will accentuate any lighting, and AFAIK using the lowest possible ISO number generally means the least graininess, which means a clearer picture, especially noticeable if you blow it up to a large size, and particularly on blues I've noticed, but I might just be imagining it.
Anyway, for the shots it sounds like you're after, as mentioned don't use the flash. Pick somewhere with fairly good lighting, shopping centre carparks are a good start. It sounds obvious but make sure you park so that you have light on the bits of the car that you want to be brightest, generally that'll be on the side(s) of the car facing the camera but it depends on the shot you want. Experiment with it, the car has wheels, if you need to move the car to get the right lighting then just do it! Leave the shutter open for quite a few seconds, I generally use 15 seconds and leave the aperture on auto unless I'm going for a specific depth of field, that might be right or wrong but it seems to give me the results I'm after. FYI, the wider your aperture is (or the lower the aperture number is, eg F1.8 is wider than F14) the shorter you'll generally want to keep your shutter open for - a wider aperture lets in more light.
It really gets fun when you start to try new things

What I did the other day (based on ideas from one of tangcla's threads, the man takes a nice photo!) was to leave the car parked somewhere dark, point and focus the camera, then leave the shutter open for a period of time and "paint" the car with light from a torch (one of those 10 million candle power things). It did the job alright but next time I'd like to try a much smaller torch - the one I used cast a very broad beam and was really bright, even on the lower setting. Maybe one of those LED maglite-style torches would be the go. Anyway, I've also tried leaving the car parked somewhere with a fair amount of light, and used the torch to brighten the darker areas. I had reasonable success in both cases - the photos weren't exactly mind-blowing but they proved to me that it was something I could improve on with some more practice... and I get to use my enormous torch
Edit: Oh btw I use a Canon 400D, the 18-55mm lens is generally fine for what I want to do, but spending $150 on the 50mm prime (ie no zoom) F1.8 lens is money well spent. The depth of field when you open it right up is really short, if you don't overdo it you can get some really nice effects from it. The F1.4 is shorter again but much more expensive :p