I found Marc Blake's book 'How to be a Sitcom Writer' very useful. Also BBC Writers Room pages on the main web site have sample scripts you can download to give you an idea of format/length structure etc. They also have, I think it's still there, a formatting tool to download, although I have to admit to never managing to get it to work, but I'm no Word expert which may have something to do with it.
Start with the characters. Who are they and why are they forced to be together. Why will we like them and why don't they like each other. Or do they like each other too much?
Then, move onto the story. What happens? Don't tell the audience show them. Bear in mind what type of show you intend it to be. Is it single camera, shot like a movie in a few locations, or in front of a studio audience? This may dictate the number of locations and limit the number of exterior shots you can have.
Then, make it funny. But not a list of jokes. Ideally every line should tell the audience something about the character, drive the story forward or be funny. If you can make as many lines as you can be all of these then that would be even better.
Hope this helps.