The first Oxford English Dictionary citation is from a January 1983 Washington Post article: "Gross me out, I mean, Valley Girl was, like, ohmigod, it was last year, fer sure! I mean, get a life! Say what?"
1986: Appears in Baby Anger page 48 as "Get a life, people of New Jersey!"
1986: The phrase was used by actor William Shatner in his appearance in a December episode of Saturday Night Live, in which he shows up at a Star Trek convention and implores a group of Trekkies who are obsessed with the details of Shatner's life to move out of their parents' basements and "get a life."