"A" is for "astronaut," and it also stands for the A's you should be getting throughout your academic career, starting with kindergarten (hey, you can slack off in preschool, but that's about it). The path to NASA is cutthroat and, at a minimum, nerd-quality grades, especially in math and science, put you in the running.
If you want to pilot the space vehicles of the future, a military academy may be your ticket—Air Force, Army, Navy, take your pick. You'll earn your wings and get lots of flight time and technical training. Mission specialists, the Earth-bound astronauts, typically should have an area of expertise in Earth or space sciences, or engineering, that involves NASA missions. A couple of internships at NASA would help. You need the basics, though: a college degree in engineering, science, or math.
E=mc2 to you, too.
And, there's more. You have to be in tip-top physical shape, and it's more than being able to squeeze out a few pushups. You can't be tinier than 5 foot 2 nor shoot past 6 foot 3. They're looking for pretty average (but very fit) body types. Not a lot of linebackers are astronauts. For all you pilot wannabes, you have to have good vision and 1,000 hours of flight time in a jet.
Next, you must qualify for and endure astronaut training in Houston, Texas, which would consume two solid years of your life. You'll be learning techniques designed to acclimatize you to space capsule, zero-gravity, and weightless conditions— like treading water continuously for 10 minutes while wearing a flight suit, dealing with emergencies in high- and low-pressure atmospheres (bouncing off walls 101), and parabolic maneuvers (weightlessness and, usually, vomitus 101).
If you make the cut, expect two to three more years of training to learn the ins and out of operating the international space station, the place to be in the orbital universe. You'll be sharpening your skills dealing with robots and other space station systems; you'll even be learning Russian. Yes, many of your comrades in space will be Russian, because the only way to space is to catch a ride with the Russians. "здравствуйте!" (that's "hello" in Russian).
You'll also spend much of the two years flying back and forth (on Earth), putting in time at astronaut training camps scattered around the world: Star City, Russia, Baikonur, Kazakhstan, Tsukuba, Japan are among the astro-hot spots.