Getting a car to safely leave the ground is tough. You have to have an appropriate place to do it, and know how to pull it off without wrapping your car around a tree when you land. The Team O'Neil Rally School put together a quick video showing you how to handle different kinds of jumps in your car, and how to keep it moving after you've touched down.
The most important aspect of hitting a jump in your car is commitment. If you're not accelerating at a steady pace and begin to lift off or brake, there's a chance your car could nosedive, causing it to land grille-first into the road. How fast you should be going depends on the size of the jump, as well as your car's weight balance. A front-engine car is more likely to nosedive than a rear-engine car, for instance, because a lot of the weight is at the front of the car.
If the jump you're approaching goes through a turn, you should already have the car pointed through the curve before you take off. If you don't, you'll land facing the wrong direction, and might not have the time or space to correct yourself when you're back on the ground.
As your confidence starts to build, the jumps will get bigger and bigger, and everything will start to happen faster. Odds are you'll never land pointed exactly where you want to go—it's tough to line up any jump perfectly. So quick reaction time is key once you regain with the ground.
This should go without saying but, don't try this on your local back roads—if you want to learn how to jump a car safely and correctly, find a closed course with lots of open space and runoff. Or better yet, just and let them teach you. This way you'll learn it the right way, and you won't put any unnecessary wear and tear on your car.