First of all, the younger you start the better. If you are still in high school, now is the time to start. If you're older, that's O.K., but do not procrastinate.
If you have never flown an airplane, go out to the local airport and take an introductory ride. These are usually fairly inexpensive, and you will get a chance to fly the airplane. If you don't like it, stop here.
If you're like most people, you will enjoy that first ride. Although the flight school will probably try to persuade you to start flying lessons right away, it is better if you don't. Instead, see if they offer a ground school course. If they don't have one themselves, they may know of one. Ground school is where you will begin learning the basics of the subjects mentioned above. If the school does not offer ground school, or if the next ground school course does not start for several months, pick up some books and get started on your own.
There are quite a few books that are written for beginners. One of the better ones is the Student Pilots Flight Manual by Kershner. The FAA also has a good collection of books.
Now comes the real test. If you enjoy what you are studying in ground school (or in the books) you will probably enjoy flying as a career. Everyone likes flying the airplane, but being a professional pilot requires a lot of study. If you don't enjoy the subjects, flying is not for you.
While most of todays airline pilots came from the military, it is not the only route. By earning your flying ratings and acquiring the experience through private lessons and later by working as a pilot for small companies, you can get the experience you need to apply to a large carrier. However, if your goal is to fly for an airline, you should strongly consider the military route. The airlines are bound to continue to prefer the experience military pilots obtain.
Whether you choose the military or civilian route, you should learn to fly privately first. This start will help you in military flight school, if you choose that route, so you can't lose by taking lessons. Get out to the airport and get started. Your worst enemy is procrastination.