In college or at a university you will want to study broadcast journalism. In the meantime, see if you can land an internship at a local television station and let them know you are interested in sports (you may not be able to start in that department right away...but if you are a team player and give 100% to the needs of the station you stand a good chance at gaining some valuable exposure and experience). If there isn't an opportunity at a local television station, try a college station or even a neighborhood network.
It might sound goofy, but you can begin practicing now. Get a few friends together for a sports commentary and videotape yourself and watch the video to see how you stack up to the pros. Would you want to watch yourself for an extended period of time? Are you dressed well? Do you have a confident posture? Do you communicate your ideas effectively and passionately? Are you objective (can you see both sides of the story)? Are you engaging or entertaining to watch? Better yet, find a mentor who is in the business of sports and have them review the video with you to discuss your strengths and areas where you can improve.
I would also encourage you to join your local Toastmasters to begin refining your public speaking skills. This will help you to communicate effectively, to learn how to engage your audience and how to keep them engaged.