For me, it would be guidance for a lot of things. However, if it could be one thing, it would be how to be sociable—OR at the very least, how to be/make friends with someone.
I feel like it would have made things easier at least for some part of my life... but it's not that simple, I guess.
“Though I have always made it my practice to be pleasant to everybody, I have not once actually experienced friendship. I have only the most painful recollections of my various acquaintances ...
― Osamu Dazai, No Longer Human