The first step to teaching any dog to swim is talking with your vet. Not all dogs are healthy enough or strong enough to swim. It is crucial for the safety of your dog to be certain that this venture won't leave you with a drive to the emergency vet clinic.
A plastic children's pool is a good place to start. Begin with letting your dog get in and out of the water on his own, then move to having him get in and out of the water on your command. If you have friends or family that have dogs that already know how to swim, bring them over! The more the merrier, and your dog will have an easier time adjusting to the water.
Once you have made it to this point, your dog should feel fairly comfortable with the water. Go to a nearby lake, a backyard pool, or a pond and play with your dog near the water. Toss a ball in an area where he can still reach it without having to swim. Bring along some of those doggy friends we mentioned before and let your dog get in as he wishes. Don't force him or he may become scared and a fear of water can be a hard thing to break.
Don't forget to test out dog life vests if it seems that your dog needs a little assistance along the way!