Your car weighs much more than you, so it is vital for your safety that you prepare carefully prior to undertaking any work underneath your vehicle.
Don't use house bricks to support a car. They can crumble under the car's weight. Always use approved, quality axle stands or drive-up ramps. It is also important to make sure that you are working on a flat area, not a slope and chock the wheels as required to ensure the car is stable.
It is also recommended that you dont work alone, just in case you need help should something go wrong.