Not a broadcaster, but have worked in the sports industry and know my share of them personally. In the beginning, take anything and everything that is available. At whatever school you have enrolled at, broadcast anything that you can get your hands on - even if it's volleyball, lacrosse, you name it.
You probably won't be able to do the high profile events (football, basketball) as a play-by-play guy at first, but you can still help out on the staff and get invaluable experience as a stringer or spotter.
Away from school, get out there and look for play-by-play opportunities in high school, amateur, semi-pro, minor league sports.
Even if you have trouble finding work, get a voice recorder, a video camera and another pal and head to a big HS game and do it yourself. Stream it on the web.
Here's a link to a guy here in San Diego who dreamed of becoming a broadcaster, he started televising major HS games and streamed them on the web.
There are millions of different possibilities out there. The main thing is you have to get out there and do it. Don't let rejections stop you. You can make it big if you are persistent and relentless in your pursuit of becoming a broadcaster.