Learning to sail: Community Boating on the Charles for a cheap, casual option (a lot of informal teaching going on, I think it's less than $200 for a membership for the summer). Courageous Sailing in Charlestown, or there are a couple on the Boston Harbor starting from downtown if you want something on the ocean.

Building a boat: Wooden Boat School in Maine.

If you get your hands on a boat, I'd be happy to show you some stuff. I'm far from an expert sailor, but I can get by on a small boat in relatively calm water.