Remove all debris from around and on the air conditioner. Confirm that the connections leading to and from the unit are tight and secure.
Flip the air conditioner circuit switch to the on position. If your unit has a separate power switch, confirm that it is turned on as well.
Close all windows in your home. Turn on the unit through your thermostat by lowering the temperature.
Go outside to confirm that the unit has started running and does not sound like it is malfunctioning. If you hear a "grr" sound, that is the compressor having trouble starting. Turn the unit off and wait five minutes. Start it again and listen. Confirm that you hear the fan spinning. If you do not, or if the unit does not start, call a repairman.