How do i make a thing that when someone clicks on the NPC, a screengui pops up?

Okay, so what i want to add is something like this, like a shop.Here's an example.

https://gyazo.com/0152e611b5d49889043c4fc33c2ae088

How do i make a thing when someone clicks the NPC It pops that up, so i can make a talking GUI, when they click a certain thing, for an example, if they click the npc, it would say "Hello, how may i help you." then options would be "Nothing." and etc.