The following safety points cannot be overemphasized:
Always block the opposite wheels or wheels to keep the vehicle from rolling off the jack.
When raising the front of the vehicle, firmly apply the parking brake.
When the drive wheels are to remain on the ground, leave the vehicle in park to help prevent it from rolling.
NEVER use cinder blocks or stacks of wood to support the vehicle, even if you're only going to be under it for a few minutes. NEVER crawl under the vehicle when it is supported only by the tire-changing jack or other floor jack.
Always use suitable jackstands to support the vehicle when you are working underneath of it. Place the stands beneath the vehicle's jacking brackets. Before climbing underneath, rock the vehicle a bit to make sure it is firmly supported.
Small hydraulic (bottle jack), screw or scissors jacks are satisfactory for raising the vehicle. Drive-on trestles or ramps are also a handy and safe way to both raise and support the vehicle. Be careful though, some ramps may be too steep to drive your vehicle onto without scraping the front bottom panels. NEVER support on any suspension member (unless specifically instructed to do so by a repair guide).
NEVER place the jack under the radiator, engine or transmission components, severe and extensive damage will result when he jack is raised. Additionally, NEVER jack under the floorpan or bodywork.