If you simply want to become airline pilot, do not join the military simply as a means to that end. Do it if you want to serve your country but don't think of it simply as a way to get "free flight training" so you can become an airline pilot.
In all reality, it's not truly "free" and the openings for military aviators are fewer and more competitive than they have ever been. If you somehow manage to even get accepted into the service as a pilot candidate and make it through flight school, you will be committed for a decade or more. You could also get killed in the line of duty.
In years past, the major airlines drew almost exclusively from former military pilots. Former military pilots either dominated the hiring boards or they would recommend their friends who were getting out of the service and looking into airline careers. This big "squadron buddy" network was how the majority of pilots got hired.
That is not the case today. For several years now the airlines have been increasingly hiring pilots from a civilian background. According to statistics compiled from Air Inc, about 65?f new hires are civilian. This is partly the result of a smaller pool of military pilots available to the airlines than in decades past and the increase in the number of civilian trained pilots with previous jet and airline experience. Almost all the regionals now fly jets, many that go higher or are more advanced than mainline aircraft. Additionally, most pilots looking to move onto a major carrier will have several times over the amount of flight experience as a military aviator because of their regional job.
If you really want to know more about military aviators, visit this message forum: http://forums.military.com
HOW TO GET STARTED:
1. Get your pilot certificates and ratings via a flying club or school at the local airport, a university aviation program. A newly certificated commercial pilot with instrument and multi-engine ratings usually has about 250 hours. This is not enough for the airlines*. (See note* at bottom) There are also programs like the ATP Career Track that get you certified then have you instruct for them and promise a preferred airline interview (At a cost of about $39K, which is the low end that you should expect to pay to get your licenses & ratings. Other schools are upwards of $80K).
2. Build up flight experience until about 1,000 flight hours. Most regional airlines require pilots to have at least 1,000 hours of flight time, with 100 of that being in multi-engine aircraft and a Commercial/Multi/Instrument certificate. (An Airline Transport Pilot certificate is not required until you make captain.) Common time building jobs are: banner towing, skydive pilot, pipeline patrol, traffic watch or flight instruction (with the additional Certified Flight Instructor rating)
3. Get a degree. You do not need a degree in aviation, but having a degree in any subject is a requirement to getting hired on at a major airline. The regionals aren't so picky. Many think they must have an aviation degree to have an edge but when or if you are ever furloughed during your career an aviation degree is useless.
4. Upon reaching the magic 1,000 hours begin applying! Hopefully you will get hired. Learn to live on a budget as regional airline pay is not very good. After building up about 2,000-3,000 hours and "paying your dues" at a regional you then qualify for a job with a major carrier.
5. Begin applying all over again.....
* There are some airline academy programs (Delta Connection Academy, Mesa Airlines Pilot Development, Flight Safety among others) that offer a "quick entry" scheme into the right seat of a regional airline. They are very costly and do not guarantee a job, but an interview only. If you pass their program and the airline interview, you could very well be hired as a First Officer at 300-400 hours. Some are very pro this method while others in the industry are not as they feel it's "buying a job". Some academy grads will face discrimination during their career because of this.