Before you buy a car, read the following articles from the Better Business Bureau:

Know the Facts About Used Car Fraud Before You Buy
Purchasing a Car Online
Don't Fall Victim to Enticing Auctions Ads for Cars!
Don't Get Soaked Buying a Flood-Damaged Car!

Things to consider when buying a new car:
• Security (anti-lock brakes, traction control and side airbags save lives!). Check the HLDI site for crash test results;
• Reliability;
• Fuel Economy;
• Warranties;
• Operating Costs.

Tips for byuing a used car
• Ask for the car's maintenance record;
• Inspect the car in daylight;
• Test drive on hills, highways and stop-and-go traffic;
• Have the car inspected by a trusted mechanic.

More about buying a used car and its financing from the Federal Trade Commission:
Buying a Used Car
Understanding Vehicle Financing