If you have a distributor, it's on the distributor.
If your vehicle is distributor-less, then it's on the side of the engine near the oil filter.
Make sure that this is the problem before replacing it. Many times it can be something else that causes the component to go bad. Replacing it may only last until the problem re-occurs. Good luck.
〉 Answered on Apr 17th, 2008 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines