The first thing to keep in mind when buying a car part is the new part's size compared to your old one. If you are buying new 17 inch rims, you will need the new 17 inch tires as well. Make sure you are getting tires that are going to fit your car. If you aren't sure, check http://www.tirerack.com They have a search function that lets you select your car model and year and returns all tires that fit for it from the major brands. That is the easiest way if you are looking to buy new tires.

If you are buying performance upgrades, make sure they will fit in your car's engine bay. The last thing you want is to go through installing a new part and then find out that the hood won't even close!

For body kits, be sure that the one you are buying is made specifically for your car. Otherwise a lot of molding might be required which will boost the install cost.

Finally, be aware that most car parts companies have pretty strict return policies. If you mount or install anything you probably aren't going to be able to return it unless it is defective, and sometimes not even then. Some companies won't let you return it if the item has even been taken out of packaging!

Next Step: Choosing Who to Buy From