This is one of the fields where there are a million different paths to the same goal. Your specific path depends on your passions and how you want to spend your time.
For many, blogging and creating a social media presence is a fantastic way to start. It will be a long and slow process, but Sportscasting is a skill and you need to practice. At the beginning no one will pay attention, and that is perfect. You'll get better as no one as watching, and as you continue to improve more and more people will begin to tune in.
Alternatively, you could begin work behind the camera. For every on-air personality there are 30 people behind the scenes. Every production requires Producers, AP's, Cameraman, Video Editors, Sound Editors, and many many more. The great thing about these roles is they are skills that can be self-taught. Working with a free video editor is a great way to begin, and if you can eventually move on to and become an expert on Final Cut Pro you will be seen as a major asset to any production.
What these people care about more than anything is you being passionate about your work. Find what excites you and expand on it, you'll find like minded individuals which will help grow your network in the field.
Hope this helps!
As with just about every job, there's no substitute for experience. It may be as simple as watching a game on TV, doing the play-by-play or color commentary, and recording yourself. You also can see what opportunities there are locally for radio or TV coverage. This might be covering local high school sports. Some cable companies may even allow you to take a camera to a game and provide commentary for rebroadcast. Finally, some colleges have outstanding programs -- such as Syracuse University's Newhouse School, which has launched many all-time greats. Hope that helps!