Someday you're going to be somebody. - Someday you'll be successful, well known, and important.
be somebody - be successful, well known, and important
By contrast, there's the phrase "be nobody", as in "Eh, he's nobody", meaning he's not important, and he doesn't matter.
Most of us are in the center of both extremes. Extremes aren't always the best way to go. Many times they're not.