I need help with giving my ScreenGui the specific purpose I've been trying to make it do?

Details of what I have done so far up until the point i need help with:

So I made a desk, and a cash register, if a player clicks the cashregisters,SurfaceGuis, TextButton... a ScreenGui pops up, and it can also close, i have those done...

Ok let explain what I need help on though...

I'm trying to make the ScreenGui be able to search for a player in the game in the textbox i made, and when i press the textbutton (named "give item"), it will give the player a name tag that goes on their shirt

(also i want to make a system in the script where it gives numbers 1-infinity, if someone else already received for example #1, they will get the next number #2 (it's like a waiting room kind of thing).

Can anyone reading this recommend what do I need to learn and practice to do things like this?