Getting your kids in the water early is important but it also important that your child can listen correctly, and follow rules. Because of this I'd say the best age to but your kid in a lesson is around five or six years old. At the community center I worked at they had plenty of mommy and me classes. These were classes that taught parents how to play and interact with their kids in the water. I would definitely would encourage most parents to play in the water with their kid before they take lessons. Kids can learn threw fun and play, so this is a great way to do this. Once the kids find the pool a fun place, they wont want to leave the pool (hehe). Play with them a little here and there but if they have a sibling they will also enjoy it.
When you first start teaching a kid how to swim you must learn the basics. Skills like floating on your back and front, blowing bubbles, and kicking are all part of a basic skill set you need to learn before you advance to independent swimming. With the kids that young I would do songs which would help develop those skills and practicing with toys, another great thing about kids that age, they have an endless imagination. If worse comes to worse create a game with it all.