Check the industry newspapers for listings for sitcom auditions. Papers like "Variety" and others often list open casting or specific roles sought after by studios and productions.
Spend time watching popular sitcoms of the day. "Seinfeld" is generally considered to be the best and most successful sitcom in recent times, but others like "Cheers" and "Everybody Loves Raymond" may be helpful.
Practice your comedic timing by doing stand-up comedy. Many sitcom actors get their start as club comedians or writers.
Write your own bits to work on and perfect your craft. Develop your own style and approach so that you know where you'll fit in to a sitcom and what sort of character you're most likely to gravitate towards.
Show up at the appropriate time for the audition. Once you get the script, memorize your lines and add some characterization or some mannerisms to your acting.
Respond to suggestions or criticisms from those at the audition. Following the advice of the writers, director and producers will be key to working on a sitcom.