How do I make a GUI pop up when I click a PLAYER?

I'm trying to make options pop up when you click a player, but how can my make a script about clicking the player when I don't know how to put anything in the player?

I don't want the script to be exactly one specific player that you have to click, or one specific player that has to click anyone. I want everyone in the game to have the privilege to click on each other. Also, I'm going to have the GUI pre-made, so can it not be a script that is creating the GUI right then and there? You can just do a Variable of the GUI like: gui = --place where your GUI is or something like that. But your answer doesn't have to be a script, it could just be an idea of how to make the script. Or it could even be how to make the players clickable or to add a click detector in them.

Help?