Jack stands
A jack stand is a more stable support for working on your car than the jack. A good hint if you are working on your car is to take a spare tire that is mounted on a wheel and place it under your car. If all you are doing is changing the tire than a jack stand is not needed. Never ever crawl under a car that is only supported by a jack.
As has been stated above, read the manual, each car has differenct points to place the jack. I have seen what happens if you place it in the wrong position, it isn't pretty. Someone just placed a jack behind the front wheel on the body of the car. It wasn't strong enough to support the weight and crumpled the body panel.
Set the parking brake and chock the wheels. By chocking the wheels I mean placing a brick or something similar in front and behind the wheels at the opposite end of the car from where you are jacking it up. That helps to keep the wheels from rolling. Yes, I have carried bricks in a car for that reason. Usually only two so I could chock just one wheel though.
Once again, read the manual or at least look around where your jack is stored in your vehicle. Instructions are usually posted there also.