When you decide to clean the humidifier, always take a look at the instructions that came with your humidifier. If you bought a used one, and you can't find instructions, then you can easily look it up online.
No matter how you decide to clean the humidifier, always turn it off and unplug it first.
Most, if not all, humidifiers will require some type of vinegar for cleaning. Your instructions will tell you exactly how much to use for your specific humidifier.
Some guides will suggest that you add chlorine to the water to disinfect your humidifier. Make sure you completely wash off all vinegar because you don't want to leave any in the humidifier.