Basically in my experience (I'm 19, been doing film for a year and a half) making film doesn't so much teach you what to do as to what not to do. For example, in my first film ever I discovered to my chagrin that I should not write or act in my own film. Now I leave the writing to the people who can actually do it and just focus on the directing. And even then I've figured out that I'm not cut out for directing actors; I prefer being a director of photography and concerned myself with how the film looks.
So basically just as everyone here already said, grab a camera and jump right in. Your first few efforts might come out like crap. Eventually though you'll notice that in your later films you just don't screw up as much (less shots that are out of focus, badly framed or just plain uninteresting). You'll develop some kind of sixth sense that tells you when something is bad (but not necessarily when something is good). I think that a good film is simply a film where the right artistic choices outweights the mistakes, and as you practice making films, the odd of you making a right choice really tips in your favor.
So basically just as everyone here already said, grab a camera and jump right in. Your first few efforts might come out like crap. Eventually though you'll notice that in your later films you just don't screw up as much (less shots that are out of focus, badly framed or just plain uninteresting). You'll develop some kind of sixth sense that tells you when something is bad (but not necessarily when something is good). I think that a good film is simply a film where the right artistic choices outweights the mistakes, and as you practice making films, the odd of you making a right choice really tips in your favor.