To be a pilot for hire, you need a commercial pilot certificate. You earn your certificate by passing commercial pilot ground school and logging at least 250 flight hours, with allotted time dedicated to certain conditions and maneuvers. After you have logged your hours and passed your written ground school test, you will need to pass a check-ride. A check-ride is something like the driving test we take to get our driver's licenses. A Federal Aviation Administration (FAA) examiner asks you to plan a flight, quizzes you on aviation matters and then accompanies you on a flight. As in a driver's license test, the examiner requests that you execute certain maneuvers and directs your flying throughout the entire flight. If everything goes well, the examiner issues you a commercial pilot's certificate.
Additionally, a commercial pilot needs an up-to-date first- or second-class medical certificate, an instrument rating and a multi-engine rating. For you to receive a medical certificate, an Aviation Medical Examiner must verify that you meet the health and fitness requirements to be a pilot. You need to get an instrument rating to fly with low visibility (in adverse weather and in clouds). You receive an instrument rating by passing instrument ground school, logging a specified amount of instrument flight time (flying without visibility) and passing an instrument rating check-ride. To fly planes with multiple engines (most of the planes in commercial use), you need to have some lessons and pass a multi-engine check-ride. At some point, most airline pilots also get an airline transport pilot certificate. This highest pilot certificate allows you to be the pilot in command (the captain) of a large commercial aircraft. It requires that you pass a written test, have a first-class medical certificate, are a high school graduate and have logged 1,500 flight hours including 250 hours as the pilot in command.
To be hired, you need flight experience. Your level of experience is based on the number and complexity of aircraft you have flown, the quantity and complexity of the flying you did (jet or propeller, day or night, local or cross-country, flying with visibility or flying using only instruments, etc.) and which crew positions you've held. Briefly, in the late 1960s, some airlines hired people without certificates or flight time and trained them from the ground up. This was an abnormal practice, and it is unlikely to recur. These days, a major airline hiring a pilot with a freshly minted commercial pilot's certificate (only 250 flight hours) is virtually unheard of. Most successful pilot applicants at major airlines have thousands of flight hours. Secondary airlines (regional or commuter) may have lower requirements.
Timing is everything. You could be the world's most qualified pilot, but if there are no openings for pilots when you enter the job market, a good job will be very hard to find. It's that simple. Unfortunately, timing is something we have almost no control over. There are no guarantees in the airline business. You won't know how your career went until you retire and can look back at it. Boom-to-bust cycles in the economy are magnified in the aviation industry. Bankruptcy, furloughs, airline shutdowns and consolidation have been a big part of the business for years. It can be, and has been for many professional pilots, a rough career ride with many different employers and lots of changes in jobs, towns and seniority. A wise airline instructor at my first airline job told the class, "The future in aviation is the next 30 seconds -- long-term planning is an hour and a half." These are the truest words about the business that I've ever heard.