First and foremost, go to college. Attend an ABET (Accreditation Board for Engineering and Technology) accredited university and get your degree in mechanical engineering. It is possible to work as a mechanical engineer with other engineering degrees, but if you know that's what you want to do, do it.
It is really helpful to have an internship or summer engineering job while in school, both to learn what 'real world' engineering is like and to see the applications for all the theory that you are learning in class. There are many job possibilities for mechanical engineers and having an internship will help you to decide what you like/dislike and what your strengths and weaknesses are. Some colleges have internship programs as part of the curriculum, but anyone is capable of obtaining an internship. Many large corporations have a summer program and some small companies are willing to hire students, you just have to look a little harder. I personally had two great internships, one with Maritime Applied Physics Corporation in Laurel, Maryland and one with Air Products
Here's a picture of a recently completed bus for False Cape State Park,
designed and built at Maritime Applied Physics Corporation.
That's an obvious one.