Solutions of Klein-Gordon-Equation

[plshelpme]

Hey there! I am currently in highschool (plus I'm pretty dumb lol) and I need help with the Klein-Gordon-Equation. I need to show that a plane wave:  Ae^i(Et-pr) is a solution of the equation. Yes, I know I just have to plug it in, but I don't know how to use operators and I don't know how I should get the relativistic energy momentum relation from it. If you could help me I would very much appreciate it, I'm currently a little (haha...) desperate... Please solve it step by step so that I can understand it.

Thank you very much!

Edited November 30, 2018 by plshelpme