You need to provide more info about the state of your car. How many miles, what kind of maintenance (if you know) has been done, how does it run, etc? In any case, start with a good tune up. Make sure you've got a fresh set of plugs and that the cap, rotor, and wires are in good shape. Run some fuel system cleaner through the tank. Also, if you don't know that the O2 sensor is fairly new, replace it. To save money, search the archives for how to adapt a generic sensor to your car. If after this you still don't pass, then start verifying the condition of the various sensors, such as the engine coolant temperature sensor. Lastly, if you still don't pass, it's probably time for a new catalytic converter (unless it has recently been replaced.