Theater is the backbone of acting. A high school theater class or a community theater center are great places to start. There you can learn the basics of stage technique and hand and body movements. Just like a weight lifter has to go to the gym to work on his skills and keep them up, an actor has to do theater to work on his skills and keep them up. If you don't want to start on stage right away, acting classes are available.
Most schools have some type of drama class that is available.
These are also great places to start. They teach the basics as specified above but in a different atmosphere. Acting classes are very specific in what they teach also. Instead of teaching you everything on stage at once, an acting class takes one section of theater and breaks it down into smaller parts so you learn more. Acting classes are great places to start for those who suffer from stage fright.