As with most things, there are two ways to learn to sail, in a school and by doing it. And as with most skills, the very best way is to learn is by doing both. Here’s what I would recommend.
First, do some research at your library and find a good book that gives detailed sailing instructions for newbies. A good book will have diagrams and pictures that will show you the ropes instead of just telling you about them.
Now that you have a basic understanding of how a boat goes, you are ready for a school and/or hands on training. A good school will mix some classroom training and theory (for which you will be well prepared because you read the book) with a lot of on the water training. Schools vary widely in terms of how and what they teach as well as the qualifications of the instructor. I highly recommend asking around about the different schools as well as a personal visit before enrolling. If you want to sail small boats, a school that trains on a 40 foot boat is probably not for you and the opposite may apply as well. Personally, I believe that all sailing skill are transferable. Learn to sail on a small boat, the skills required for a larger craft will come easier to you. You will also want a small class size so that you can make sure you get lots of sailing time as well as personal instruction.
If a school isn’t for you, try visiting a sailing club where they hold races. Most racers are constantly looking for crew and nothing perfects sailing skills better than competition. Try to find a patient “skipper” who will explain things to you. Be absolutely sure to inform the skipper as to the level of your experience. Some will be very interested in winning and sailing with them as a novice can be a very frustrating and possibly unpleasant experience. Others want to enjoy the race and have fun. These people may not be the very best sailors but will definitely make the best and most pleasant instructors as well as allow you to perform the most duties. If you hit it off with the skipper and enjoy the racing, you may be able to make your crewing a regular arrangement. One season of racing and I guarantee you will know how to sail.
No racing available where you live? Then either try to make friends with someone who owns a boat and will take you out several times or rent an boat. If you rent, get the smallest boat available and stay near the marina until you develop some competence and confidence. Getting someone who already knows how to sail join you will make the experience a lot more fun and greatly reduce any beginners anxiety. Do not take a date, your spouse or children the first couple of times you go out. You will need to concentrate on doing the sailing and the distraction will not be helpful.
Sailing is a “doing” activity. Theory is great but nothing is better than experience. As the saying goes, “The elegance of theory will never withstand the messiness of reality.”