What you'll need to do is brave your attic. If your system's designed properly, there is a primary condensation line. This is the one that you need to clear out. If it is designed properly there will be a clean out hole that you can open to expose the drain. Pour a mixture of hot water and bleach into the hole. This will eat up the algae and free up the line. If it is so far gone that it is really stubborn. You may need a plumbers snake. Run it down the line as far as you can. In most cases this will probably clear the drain line.
If this is clogged then the water will be coming out into this pan in which your condenser sits. And then it's draining out of the back up line. If the emergency pan were to overflow, you are going to have water seeping through your ceiling, and you're going to have thousands of dollars in damage on your hands. So it's very important to take care of this as soon as possible.
There are other types of maintenance items that should be performed on your A/C unit, but these should be done by a professional air conditioning service. NEVER open your air conditioning unit. Opening the unit can be hazardous and even fatal without the proper knowledge of the hazards. You may call any air conditioning service to have yearly maintenance done. Consult your owner's manual for more information.