If you have always wanted to fly the big planes, then you are going to need to spend a lot of time working yourself up to being able to fly those planes. You can't just walk up to one of the big airlines and tell them that you want to start flying their big 737s. They are going to want you to have plenty of experience and flight hours under your belt before they even consider letting you get into the co-pilot's seat, let alone the pilot's seat. So, what is it going to take for you to get behind the stick of one of those great big planes?
Learn How to Fly
It seems like a pretty obvious first step, but there you go. You aren't going to start out learning how to fly the big planes, you are going to start out on the smaller, one engine prop planes. There are lots of reasons for this, but think about when you learned to move around. You learned how to crawl before you learned how to run. Learning to fly a smaller airplane is learning how to crawl, and gives you the basics that you are going to need to know when it comes to flying. There are different ways you can learn to fly. One is that you can go to a commercial civilian flight school or you can join the military. If you wish to join the military to get training, you may want to take some private lessons before you join, just so that you have some skills under your belt and you will be more likely to get into pilot training once you have joined whichever branch of the military you wish to join.
Learning to fly is more than just getting up into the plane and driving it around the sky, there are a lot of other things that you have to learn as well. That's what ground school is for. You will learn things like the physics of flight, aerodynamics, and other similar things. There is no way to get around going to a good ground school, it is nothing that can hurt you, and the more learning you have, the more likely the big airlines are going to hire you.
If you want to learn how to fly big airplanes, you need to start flight training by flying the little planes and work yourself up.