Try hacking your car with some white paint! White objects reflect light, while black and other colors tend to absorb most of the light. During the summer, when the sun is out, your car can heat up a lot. The more light it absorbs, the more energy (and consequently heat) is contained in the car. This can make it rather unpleasant to be in dark car. White cars will generally stay cooler than other kinds.
If you're just now buying a car, consider buying one pre-hacked. Most car companies will sell you a white or light-colored car. If you can't do this, most automotive maintenance shops will repaint your car. You can do it yourself if you know what you're doing, but keep in mind that water based or latex paints aren't recommended on metal surfaces; the easiest way to paint your own car is with specialized auto paint or spray paint.
Dark colored car fans might despair, since this can't be used in conjunction with the "Dirt and other marks are basically invisible on a black car" hack. But there is an option which preserves the best of both worlds. If you really can't part with your dark-colored car, consider investing in a light-colored tarp and sticking it in your trunk during summer months. The tarp should be large enough to cover the car completely. When you need to park in a spot that isn't completely covered, put the tarp over your car. You may see even better results with this "white tarp" method than with the "white paint" hack, but the tarp will take some effort to find and may cost on the order of 10-20 USD. Also note that there are some specially made "car covers" which are just tarps designed specifically for this purpose, so you might want to look into that, though they might cost a bit more than the equivalent-sized tarp would.