Shoreline Sailing Lake is a great place for beginners. They have lessons in the Spring/Summer/Fall and have rigged and ready rental gear (mostly beginner gear, a few nicer rigs for intermediates). It's a small lake and there's lots of traffic on summer weekends, including sailboats and paddle boats. On the weekends the parking lot fills up by early afternoon.
Great place to bring the family as there is lots of open space, a cafeteria right on the water and it's generally warm compared to other Bay Area sites. Generally less windy than most other Bay Area sailing sites, peak wind is usually 2:00pm - 5:00pm.