If I want to pursue a career as an astronaut, what is the best way to go about doing this?

Become an airforce officer who is combat pilot trained.

Get yourself a degree in science or engineering, specializing in some discipline that would be useful in space.  It could be helpful, but is not necessary, to learn to fly an airplane.  If you can get yourself an appointment to the Air Force Academy, you can probably do both.

This will be a hard question for anyone to answer since I doubt many people researched this role. I suggest looking up Google, there's a lot of info available, you just have to search for it. Hopefuly you'll become one of the most known astronauts of our time, go get em!

Join Civil Air Patrol, Jr ROTC, ROTC.  Take all the science courses you can, math, chemistry, engineering, computer.  Go to college and get an engineering degree.  Go to a military academy if at all possible.  Become a military pilot.  

Astronauts aren't just astronauts, they are pilots and engineers.  You need a scientific speciality that would make you valuable on a space mission.  You need to be healthy and strong.  You need to be able to handle stress and not panic in an emergency situation.  

And that's just a start.

I was recently in London at the national science awards and we went round the science museum where we attended a lecture from some wannabe astronaut who was telling us all about the space travel and the sciency things which i won't go in to. Near the end he said around your age (15) is when Nasa start picking up their new astronauts.
           Because it takes so long to train you, you start at 15 and will probably fly at 40. So if nasa havent contacted you, or any other space agency then unlucky mate, or you could be the guy at the science museum dressed up in an astronauts suit. He looked pretty sad to be honest. You don't do college, you do training and you are just trained to be an astronaut, obv with science an stuff aswell. But its probably too late, and your not American! are you?

All these answers are good, but to be more to the point TANG! JUST TANG drink it as much as possible. It has special astronaut boosting ingrediants.

The ABSOLUTE FIRST STEP:  Get tested to see if you're colorblind!  I am (and no, that does NOT mean I see black and white!) and therfore I can never be an astronaut.

There are actually several things that colorblindness limits you from doing to some degree or another; electrician, pilot, certain specialist jobs with the military...I mean, if you can't distinguish the colors of wires, do you really want to be messing with electricity?

Being colorblind is currently untreatable, but perhaps in the future there will be some way of giving someone with color deficient vision a close faccimile to what "normal" people see.

Fact: It is very rare (if not impossible) for women to be colorblind, BUT it is the woman who passes the gene on to the man! :-)

I really hope I haven't smashed any dreams...just telling you what I know.