In the United States the United States Tennis Association offers a free training program that is available in most communities. In addition there are low cost municipal programs that allow both children and adults to learn the basics of tennis as well as the rules to allow them to be able to play the game.
When you sign up for tennis lessons, make sure that you are signing up for a program that provides you with progressive lessons so that once you learn one area you will move to the next in succession. This gives you the best ability to learn to play the game.