This question is asked many times a day around here. I'll give you a quick summary, but search the questions and you'll get a lot more answers.
The biggest thing you have to do is love to fly. An international route on a 747 will come with at least 20 years of experience in the industry. If you don't enjoy the journey, you will never make it to that point.
For flying a large international plane like that, a 4 year college degree is a must. It doesn't matter what the degree is in. Business, management, physics, meteorology, and aviation are all fine choices.
For flight training, the basic options are through a local airport, through an aviation program in a university, or through the military.
Military flight training is the least expensive, but you still need a college degree to be commissioned as an officer in the flight training program. Enrolling in an ROTC program will help.
The other options will cost at least $50,000 (not counting college tuition). Be ready for many years of time building and low paying jobs.
Again, you must love to fly whatever you happen to be flying for your journey to a 747 to be successful. If you won't be happy in a Cessna, a King Air, or a regional jet, this probably isn't the career path for you. If that does sound appealing, start with an introductory flight at a local airport. You'll know if you're hooked or not!