Get a couple of rides on someone else's boat. Ask questions. Observe how things are done.
You can teach yourself the basics. Read the wiki on how to sail. Watch a few youtube videos.
Rent a small boat like a Sunfish and practice on a few relatively calm (5-10 mph wind) days. Slowly build up until you learn to hold a close-haul tack "hiked out" so far that you have a foot on the daggerboard UNDER the keel
. (those were fun times)
Flip the little boat a few times... they pop back up easily enough. You'll learn how it feels when its about to go over.
Learn to prevent the flip by letting out the main sheet (rope controlling the sail) and/or turning to unload the sail.
Read up on what it means to "tack" or "Jibe" (Gibe) Tacking is pretty safe. A Jibe can break things and kill people if done wrong.
On the Sunfish and similar a bad jibe can knock you out and over the side... Wear a life vest.
Get lessons if you go past appx 14 ft boat on the lake in fairly calm water
RENT a few boats to see what you can handle and if you really like it. Too many people buy, sail once and leave the boat tied to the pier to rot