There are not many prerequisites to become an airline pilot in the United States. While a college degree is an asset, it's certainly not required to obtain a pilot's license. In the past, most airline pilots were trained in the military, but that's no longer the norm either. For starters, it requires a 10-year commitment to the Air Force, and many people who have their sights set on becoming an airline pilot simply want to fly, and not go through military flight training or live a military life.
Most people who want to become an airline pilot are looking for a civilian license. This type of license can be obtained from a private flight school or from a college that offers a degree on aviation. Both options cost a lot of money. You can start with a basic private license, which requires a minimum of 40 hours of flight training, or go directly for a commercial license, which starts at 250 hours. Most private schools charge $80 US Dollars (USD) and up for one hour of flight. Classroom instruction, flying charts, and books are all additional.
After the required hours are finished, those who want to become an airline pilot must pass a check-ride, the equivalent to a driver's license, which consists of a quiz, an oral test, and a practical test. While a pilot's license lasts forever, you'll need a medical certificate to work for an airline, and this is only good for 6-12 months, depending on the type of plane you fly. Since 9/11, all pilot licenses in the United States are now given under the permission of Department of Homeland Security (DHS), through the Federal Aviation Administration (FAA).
After you obtain your pilot's license, you need to get flight experience before any airline would consider hiring you. Most airlines would not consider any pilot that has under a few thousand hours of flight in his or her charts. To become an airline pilot for commercial planes, you also need an instrument rating, which allows you to fly in bad weather conditions, such as below VFR (visual) minimums. You may also need a multi-engine rating, which allows you to pilot all planes under 12,500 pounds (about 5670 kg).