You can't just walk into a television station and get hired as a sportscaster; for some reason they need you to get some kind of education and experience first (go figure). You'll want at least a bachelor's degree in broadcasting or journalism, and you should probably try to work at the college's team station if they have one (and if they don't, you're probably not at the right school for your sportscasting dreams).
You'll also need to put in your time interning at smaller stations or news organizations to build up some cred—in the broadcast world, experience is everything.
You'll also have to demonstrate an expert knowledge (obvi) of just about everything having to do with a ball, puck, and/or racket. When starting out, it's important that you specialize—or own, if you feel like marking your territory here—a particular sport. You want to become known as the go-to sportscaster for whatever overpriced game you happen to be obsessed with.