Fresh water or salt water? A lake, river, or open water? Single handed sailing or with a crew? I learned to sail on a small mid-west lake, in an 8ft Optimist pram. One sail, light weight. Lots of fun. The old Sunfish was also fun, but I don't think it's a great boat to learn on. If you want to buy something that you won't outgrow in a season, try this:
It can be sailed quite comfortably, solo.