Be talented. If you're an incredible musician or singer, your abilities should speak for themselves, and you may go far. Take classes, practice or rehearse frequently and for hours at a time, and try to diversify (singers who also play an instrument have a better chance of success than those who do only one or the other). Then get out there and audition as much as possible, get yourself seen online or in public, and it may not hurt to schmooze a bit either. There aren't too many superstars who didn't kiss a little booty at some point early in their careers.