Understanding exposure and composition is more important than WB adjustment. You can adjust WB, hue, saturation, etc, in post processing (Lightroom, PS, GIMP, etc.)
Using any setting other than Manual (M), will give the camera input into the exposure, so start by making your task easier, and use P,A, or S modes, depending on what you are shooting.
Exposure is controlled through 3 things: how long the shutter is open, how open the aperture is, and how sensative the sensor is to the light (ISO).
The higher the ISO, the more sensative the sensor is to capturing light, but the additional sensativity comes at a price, which is increased noise.
A is Aperture-priority, which means that you set the aperture, and the camera calculates what shutter speed and ISO is needed. The numbers run in reverse (smaller number means bigger opening), and you should think of it like a water pipe - the larger it is, the more light it can collect in a given amount of time. Aperture also lets you control the depth of field.
S is Shutter-priority, which means that you set how long the shutter will be open, and the camera calculates what aperture and ISO is needed. Higher shutter speeds help you kill ambient light (when you are using a flash), and capture sharp images of things moving (like cars and children). Slow shutter speeds let you collect a lot of light, and use lower ISOs for less noise, or low light areas. Slow shutter speeds can also let you capture blur to display motion. The faster the shutter speed, the longer that the aperture has to be open, to collect enough light for a correct exposure (or you have to raise the ISO).
P is Program mode, and lets the camera figure out aperture, shutter, and ISO for a correct exposure. It works well for most things, but you still have to keep in mind that it doesn't know exactly what you have in your mind - it doesn't know exactly how you want the shot to look.
I suggest that you read "Understanding Exposure" by Bryan Peterson, and go through some of the online information. A good place to start is the Adorama Learning Center: http://www.adorama.com/alc/
For shooting cars at car shows, I've found that I use 3 lenses on my Nikon the most:
35mm f/1.8 - Sharp photos from a prime lens, I can control the DOF, and excellent low light shooting for night meets. I generally only carry this one if it is a night meet.
10-20mm - Too slow for low light, but good during the day. With a wide angle, almost everything is in focus, so DOF is always going to be longer. Canon makes an excellent 10-22mm, and if you like to get up close, and take twisted perspectives, then you have an excellent option.
18-55mm - The kit lens, like you have. I can easily just shoot this lens during the daytime for cars. You can't get the great DOF that a pro lens has, but it's light, and sharp, and 18mm is good enough 90% of the time. You already have it, so just shoot it, and enjoy it.
The one that I rarely use at a show: the 55-200mm. The reason that I don't use it as much is that you have to get farther back, and people walk into the photo. You can get good photos from it, and it's light, but you have to have patience with people. Chad has the Nikon pro version, the 70-200, but he also has the patience to wait to get the shot.
One thing to mention on getting low to get the shot - it looks better, but you have to make certain not to "cut the roof down" - make certain that you still get all of the car in the shot that you want. Sometimes you need to get a little farther back to get the low shots. I've made the mistake of getting too close and too low several times.
"Racing is life. Anything before or after is just waiting." - Steve McQueen