Take classes and make films!
I'm an aspiring filmmaker and that's just what you have to do. It's the best way to learn, being hands on. Write a story outline, then a script (check out celtx.com), and then put it together. Find some friends to act and find a good time to shoot. If you don't have many friends interested in performing, recruit family members or just act in it yourself- that's what I tend to do considering many high school students don't know how to act/over act.
Invest in a decent editing software, a nice camera (if you can, otherwise, don't worry about it), and a tripod. Unless there's a specific effect you're going for, shaky-cam isn't the way to go!
Read scripts. READ. Watch movies and dissect them. Find what you don't like, find what you do. Develop your style. Read books- as you read you can apply your visual style and create a film in your head as you go. Write, and write a lot. Watch more films. Ditch the chick-flicks and explore everything. I found out that I love Quentin Tarantino and Martin Scorsese, even though most girls my age like Mean Girls and Bridesmaids (both fine movies). You've just got to work hard and do what you love.
Filmmaking is much harder than many give it credit for. You have to have passion and drive and a unique sense of creativity that will set you apart from the rest. You have to know what you want and not stop until you get it- you can't take no for an answer. You have to be able to see the world in a different way from everyone else in order to bring spark to your film.
Best of luck!