Here's a collection of great sing a long songs when you haven't been playing long - Good Luck!
John Denver, Beatles, Creedence Clearwater Revival, Avril Lavigne, White Stripes, Bob Dylan, Dusty Springfield, Runrig, Mike Oldfield, Paul McCartney, Scott McCanzy...
Grateful Dead songs
Grateful Dead and The Grateful Dead
All of the songs that have been rehearsed, performed, and/or attempted by the Riverside Ukulele Liberation Front at least as far as I can remember
Daniel Boone, Bruce Springsteen, Traditional, Tally Hall, John Lennon, Tom Waits, David Bowie, Garth Brooks and Woody Guthrie