It obviously helps a lot to have beat writing experience, and experience covering the sport you may be applying for, but it isn't always mandatory. If you can write, and have experience breaking news and writing on deadline, all that helps, too. Different publications and editors look for different things.
One suggestion: If you know which beat you want to cover, find a publication that covers it and reach out to the editors and writers to make a connection. Try meeting them in person, if you can, then build a portfolio of work while cultivating those relationships and one day it may pay off.