Shop local. When shopping for a car online shop local. Check your local newspapers website, craigslist, and kijiji. Sometimes there are new sites that allow people to post local classifieds doing a "local classified" search will help you find them. Browse through these sites until you find a couple good possibilities. Make sure you check the cities they are located in if travel is a concern.
Check the vehicle out. If you do not know much about cars definitely bring someone who does. Take your time, fully inspect the car and ask questions.
Do research on the car. Check out the Kelley Blue Book value to make sure the car is not priced to high . Also, check to see if there has been any recalls for the car. When I purchased my 2001 Honda Odyssey I didn't know that the transmissions go bad prematurely. If I would have known that I could have negotiated the price down or turned it down all together. (see resources for link)
If everything checks out get a carfax report. With the report you will get a accident/frame damage check, title problems including salvage, junk or flood, the number of owners, service Records and more useful information. For twenty bucks, knowing is better than not.