Actually, for me it was two things, both learned at the same time. A friend and I bought our first boat, an 11 foot Glen L cat, in the dead of winter. I spent the time until the thaw reading every book about sailing in the local library. When spring came, I knew no more than when I started, except that I was more confused. I knew all about mast
bending and leech lines and racing
tactics but knew nothing about how to sail.
We went out as early as we could to a local lake. There, who did we meet except the guy that we bought the boat from. He had built our boat and came to see his baby off. We launched and couldn't move an inch. There was just no wind
. We knew that. The PO didn't, though, as he sailed in circles around us. We came home deeply depressed at our lack of sailing ability.
Back to the library! I found the Cub Scout sailing merit badge handbook. It's the best treatise on basic sailing I have found to this day. Here are the two things. First, face the wind
. Look 45 degrees to your right and 45 degrees to your left. You can't go there. Now point the boat in the direction you want to go. Let the sail out until it luffs and pull it in until it stops shaking, the pull in a bit more.