Check your local library. Some have free-for-all chess sessions once or twice/week, mostly attended by entry level kids.
Learn the most basic rules and watch advanced players play. Yahoo chess is good place for lurkers. Study endgame routines – don’t be the guy who draws with knight and bishop.
My 5y old plays solid game after some 100 hours of practice over the past 6 months. Keeping my mouth shut while he’s playing other kids is hard.