You'd have to pick a sport and focus on it. In many of the major sports, these athletes spend hours and hours everyday honing their skills, and so few ever actually make it. A few months ago, I wrote about how few baseball players ever get a chance to play in the Majors on my website - http://www.danstake.com/2009/07/everybody-was-prospect-look-how-most.html
That said, if you do work hard enough and you have enough athletic ability, you never know, it could happen. Just make sure you spend enough time being a kid and doing well in school as well, so then you'll be a well-rounded person.
