Whenever you work under your car, make sure it's properly supported! This does not mean cinder blocks, blocks of wood, or anything jury-rigged. The proper method is the use a jack to lift the car, and then support it with jackstands. A jack by itself is not intended to support the car, only lift it.
On the bottom of the car, just behind the front wheels (and in front of the rear wheels), there are four indentations for jackstands. This is where you should place the jackstands to support the car.
On the front of the car, you can raise the entire front end of the car by placing the jack under the front crossmember. It passes right under the engine, and has two bolts holding it towards the front of the car. There is an indentation/hole where the jack can rest. Use a block of wood between the jack and the crossmember to avoid scratching and damaging the metal.
The rear jack point differs for FWD/AWD vehicles. On AWD vehicles, you can place the jack under the rear differential. On FWD cars, you can place the jack under the rear crossmember (between the floor pans). Again, use wood to protect the metal.
Here are some illustrations on proper jack points:
