How do I make GUI that has functions like onClick where you can add click functions?

Basically I'm trying to make a game like your basic hangout game Where I add these gems to obtain, Basically I'm also adding a gamepass that has "Trading" with it, where you can exchange your gems for fake robux or currency that you can buy items with and trade with eachother, etc, I'm really wanting help on scripting help with GUI's and onClick functions.