Do you know vaguely how to play? If not, I would choose a book because that will help you progress easily. If you do, Minuet by Bach is fun. That was one of the first ones I learned, and I can't read piano music. YouTube videos are helpful if you learn that way, and sometimes a keyboard will teach you, so you can learn easy ones from there.
Happy birthday, silent night, deck the halls, Mary had a little lamb, Amazing Grace, the muffin man, London bridge, when the saints go marching in,if you're happy and you know it, lightly row...those were easy for me to learn at least.