When designing a car, you might want to begin, well, with the beginning. Ask yourself "what am I making?" and "What market am I aiming for?"
Something else important to note is the year you're designing in. The market will be wildly different in 1960 than it will be in 2010, so sometimes (especially when playing campaign) it can be a good idea to make cars you're certain will sell well.
Of course, to begin making a car, you need a body. There are plenty to choose from no matter the year, and they fit all kinds of different markets. If you want to expand your horizons, there are also mods for additional bodies. Once you've found a body you like, you can begin to select materials. Different materials have different ups and downs (as you will see by hovering over them), so choose whatever you think suits your car best. The same goes for suspension - for example, a heavy-duty truck will be much more capable if fitted with rear leaf springs than, say, dual wishbones, as the leafs can hold more weight comfortably. The engine placement can also vary from body to body. Not all bodies will support all engine placement options. A mid-or rear-engine car is more likely to strike a chord with the sports car market, while a front-engine car is more "regular" and will also likely have a higher production volume.
After figuring out a baseline, it's time to move on to the engine. There are many variations of engine layouts to choose from, anything from a tiny economical inline-3 to a massive, gas guzzling V16. Different markets will prefer different engines - of course an economical city car market will not be interested in a 7-liter V8 - it's probably better to use something like an I3 or I4 engine for that. The opposite is true for performance cars, the market is more likely to be interested in a powerful engine. Be careful though, not many markets are interested in a 3000-horsepower V16. Still, don't be afraid to experiment with different engine layouts, parts and tuning - knowing how they affect the car is a very good thing to know.
After making the engine, it's time to move onto the trim - the actual designing of the car. I recommend finishing the trim and at least beginning to tune the car before beginning work with fixtures, as that will give you a good idea as to what the vehicle is capable of. Even so, the vehicle can often be tuned differently. A car with soft suspension will likely hit the luxury market moreso than the same car tuned to have hard suspension - so figure out what you want to aim for, and weigh your possibilities.