Condensation on windows occurs when humid air hits colder glass. To get rid of condensation on a skylight, you will either have to reduce the humidity in your house or raise the temperature of the glass. While there are some simple solutions to reducing condensation, many long-term solutions will require hiring a professional, unless you have roofing or window installation experience.
One of the quickest (but very crude) ways of reducing condensation is to point a fan at the skylight. This may push enough warm air onto the glass that it warms the glass itself. This should be used only as a temporary measure, and it may not work at all if the glass remains sufficiently cold for condensation to form.
Reduce the humidity in your house. This can be done by reducing the use of humidifiers, venting washers and dryers outside, covering up crawl spaces with plastic or running ventilation fans in bathrooms and kitchens. If you are unable to sufficiently reduce humidity, you will want to run a dehumidifier near your skylights.
Install window film on your skylights. To do this, simply measure your skylight and cut the window film to match the size of the glass. Clean the skylight thoroughly, then fill a spray bottle with water and a couple of drops of dish detergent. Spray the window, and press the window film against the glass. Hold the film there for a few seconds to make sure that it sticks. Refer to individual kit instructions for warnings and special applications.
If condensation is still a problem, you may need to hire a professional to install a second layer of glass or an outside storm unit.