Summarize this article:

Many indoor and outdoor air conditioning units will have a drain or drain line in the bottom corner of the drain pan at the bottom of the unit. It will look like a hole in the pan, and on some models, there will be a nozzle with a rubber or plastic line coming out of the other side. Make sure the drain isn't blocked or plugged so water and cleaner can drain as you clean the coils.  If it has a drain line that seems to be clogged, pour a 50/50 water and bleach solution in the drain pan and wait a few minutes to clear it out. You can purchase algae tablets from air conditioning specialty retailers to prevent algae from growing and clogging the drain. With the cover set aside, you can look around at the air conditioning coils and make sure there's no large bits of debris that may have worked their way into the unit. It's not uncommon for leaves, sticks or bugs to find their way into the interior of an outdoor unit.  You might want to use a shop vacuum for things like spiderwebs if you're uncomfortable removing them by hand. Be careful touching the fins, or blades, on the coil. They bend easily. Most compressors and motors in modern air conditioning units are maintenance free, but keep an eye out for dark drip marks on the bottom of the compressor housing or beneath it. That may be a sign of an internal oil leak.  If the compressor or motor are leaking oil, it will need to be repaired by a certified technician. Don't attempt to over-tighten connections on the compressor or motor, as you may worsen the leak. Be sure to use your garden hose and not a power washer, as too much force could bend the fins inside the air conditioning unit. Spray the coils down to remove any loose dirt and prepare the coil for a spray cleaner.  If the fan is still in place, avoid spraying it or its wiring directly with the hose. If your unit is indoors, you will need to use a different kind of cleaner for the next step.

Summary:
Clear out the drain line (when applicable). Remove large debris from inside the unit with your hands. Look for signs of oil leaks. Spray the coil with water from the hose if its outdoors.