Hi Kishlay,
Something to keep in mind is that when you have a dream like this, a lot of people are going to bring up the realities and probabilities involved and it will be easy to become disenfranchised. But I think... particularly for someone who is 16, that it's important to have dreams like this, and to chase them.
I would highly recommend Chris Hadfield's recent book: An Astronaut's Guide to Life on Earth. Colonel Hadfield decided he wanted to become an astronaut when he was very young as well, but he was not American. There was no Canadian space program at the time. And yet, he persisted. He became one of the top pilots in the world and when Canada did develop a space program, he was one of the handful of potential astronauts selected. He has been to space three times now, I believe, most recently in a six month mission as the commander of the International Space Station.
Sure, the odds are stacked highly against you. But this is the kind of dream that there is value in pursuing. Say in your endeavour you become a scientist, a pilot, an engineer or a medical doctor - all are constructive professions that could allow you to do a lot of good in the world.
Read the biographies of other successful astronauts and cosmonauts. They'll give you an idea of what the "right stuff" is - or at least what it has been.
