Reddit mentions of The Radio Producer's Handbook

Does your school have a student radio station or television station? If so, ask if they have sports programming, and if not, make your pitch to try to get it started.

Otherwise, spend some time studying up on radio and voiceover in general. I'd suggest looking into these three books.

It's not an easy path from what I've seen, but the ones that seem to make it are persistent, hard-working, and put in the time to improve their craft, even if they aren't getting paid for it yet.

Edit: If no on-air options are available see if local high school, regional, or university teams need held for public address announcing or anything of that sort.