one time set of values, how do you do that?

I want for each key push, a certain string gets added to a main string which i can handle later on
So if i press “k” then string = string+“1” and if i press “l” then string = string+“2”
Anyways, if i say in the start of the script: string = “”
Then for evey key push it will erase the prev string because it says yet again that string =""
so I need the string value to be a value that get deffined ONCE so that it doesnt say string = “” each time it gets an impulse

You can use a string property on the object, or use a global variable. That is putting a variable on the GameLogic module. e.g:

if not hasattr(GameLogic, 'string'): # if GameLogic doesn't have a string variable
    GameLogic.string = ""

So i could also save an UDP-Module in the GameLogic, for creating a MultiPlayerGame?

Yea it should work.

Also, you can make scripts run once by connecting them to an always sensor with both pulse modes disabled.