If your AC works fine (blows cold air at your face) DO NOT ADD FREON! Too much or too little causes problems. Take your car to a certified AC repair facility and have them drain the system and recharge it with the PROPER amount of freon, measured in lbs.
There is both a high and low pressure switch in the system. Too much freon raises the pressures causing compressor damage. Fix it soon or it gets real expensive.
IF YOU DO NOT KNOW WHAT YOU ARE DOING WITH AC SYSTEMS, LEAVE THEM ALONE!