Almost any mechanical job you might tackle may require that you are able to safely jack the effected vehicle to a position where you can either: access components under the vehicle, remove one or more wheel(s), raise the vehicle to lower vehicle components when otherwise impossible (ie. transmission). As a disclaimer, we always recommend that you read your vehicle
I would suggest that you check your owner's manual as to where to jack up the vehicle safely. If you are doing something other than replacing a tire, you should probably have a technician do the work for you.
You never work under a vehicle using only a floor jack. You must use jack stands to work safely. Again, if you are unfamiliar with how to use them, have a technician do the work on your vehicle. I've heard too many horror stories about people being crushed under their vehicle. Be careful.
〉 Answered on Dec 24th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines