How do I make my own car and sell it to the world?

Realistically, it's not an easy process. It is also a team effort. Several people come together and design different aspects and make them fit. But if you are serious, you can study math and science in school, go to a good four year college and major in mechanical engineering, and get hired onto an automobile manufacturer. Eventually you may be put in charge of a design team and then you can build it. If you are more concerned with how it looks, then study graphic design as well, and work for a body manufacturer.
It is not easy, I know, but it will be rewarding in the end.