When I first started my favorites were Look At What I Can Do(only if you know some of the notes),you should buy David Carr Glover Method for Piano Lessons Primer Level or Level One. It's perfect for learning even if you don't take lessons. I hope this helps and some really really easy songs are Mary Had a Little Lamb and Ode to Joy. I hope you continue playing piano it's such a great experience!! :)
Some easy songs to learn on the piano include Imagine by John Lennon, Moonlight Sonata by Beethoven, Mary had a little Lamb and Twinkle Twinkle little star. You could also buy a piano book that contains easy songs that you can learn how to play.