If your condensation drain tube connected to the pump is clogged check to see if there is algae growing inside. Algae grows in wet conditions, so it is a common occurrence. To clean your condensation drain tube, run a wire through the tube to clean it. This will clear out any debris. If the algae is thick you will need to kill it with a solution of 1 part bleach to 16 parts water. Pour this into the tube. Allow it to sit for an hour before rinsing the tube with water. Run your wire through again to remove any loose algae.
If there is water forming at the bottom of the air conditioning unit, your pump may not be functioning. Check to see if any of the tubes coming out of the pump are leaking liquid. If they are you will need to replace them. Condensation pumps have a pan that collects water built into them. When water collects in this pan the pump will start running. Try pouring some water into the pan to see if the pump turns on. If it does not, check that it has power. If it does then the pump may be broken.
If the pump begins running but the pan does not empty you should check to make sure the check valve, a ball like object inside the unit, is not stuck. Open up the check valve of the unit and see if it is blocked by debris.
Before turning on your air conditioner each year, check to make sure everything is in working order. Check that the pump and drain pipes are clear of debris. Disassemble the entire unit and scrub with a cloth and liquid dishwashing detergent. Fill the unit with a bleach solution and allow the pump to run for several cycles to clear out any debris you may not see.