Before writing, observe other successful comedies, whether in stand-up, film or book form. Study their form and take apart their process, writing out a diagram of the comedies' story structures and jokes. Ask yourself what these comedies do that make them successful. Carry a notebook around with you to write down whatever funny, unusual idea comes into your head.
Research topics that interest you to help build a believable story. Use this research to create an unlikely or odd point of view of a situation which could be construed as humorous. Be sure to build a comprehensive knowledge of the topic.
Read the newspaper everyday. Be updated on current events, both local and national. Cultivate original ideas from unusual stories in both the news and magazines.
Use stories from personal experience, as personal experience is the key to making original stories. Note that on sitcoms and comedy films, funny stories come from a character's personal or professional life. Consider stories from your own past that are funny and out of the ordinary. Determine whether such stories would be a good fit for your original comedy.