It depends how old you are and what your schooling is like. Most astronauts are scientists and have graduate degrees. They are also very very fit. The rest are usually ex-military or test pilots. The best course of action is to take as much math and science classes as you can, study hard and either join the military applying to be a pilot, or do graduate studies in a science area that interests you.
Those are pretty much all pre-requisites and the application to be an astronaut part I have no clue about other than there are probably WAAAY more people wanting to do it as there are slots available. Even once in the program it could take ages to actually get on a mission and then you still may never make it into space.
It is really interesting reading to go to the NASA website and look up the astronaut profiles. You can see all about their background, what missions they have been on, if any, and if they are currently training for any.