This issue is fairly well documented - I know I personally have replied in lengthy detail in at least 3 threads about it. But, at the sake of repeating myself and to help you get your AC restored sooner than later, here ya go.
Your refrigerant is low. When it reaches a certain point it will only cool the side closest to the expansion valve which is right of center behind the dash. The easiest, fastest and cheapest first step to verify this is to add a little bit of refrigerant and check the vents. If you add 1/3 of a can and the cooling is restored to the driver's side, have your system charged correctly and brought to the correct level. If that doesn't have any effect, you most likely have a plugged expansion valve. Don't continue adding refrigerant as you could permanently damage the system. It will need to be evacuated and the expansion valve replaced. They're not expensive and is something you can do yourself. You would then pull a vacuum on the system then recharge.
Take a minute to add your car's specs and info to your sig so we can have the pertinent info when attempting to help you in the future.