Controll the Board from external computer software

Ok. Lets set up an hypothtical situation:

I have my Arduino UNO connected via USB-Cable to a computer with a OS of my choise. Now I want to make a programm, that consists of just one big button and nothing else. Everytime I press the button I want the Arduino to flash an LED which is connected to it.

How would I do this? Or: How Can I do this? (Because thats something I'd like to build)

Sure you can do that. Just communicate over the serial port. Write code on Arduino to react to what you have the PC program send. What language do you plan to write the PC program in?

It's a common question. So common that there is a whole section of the forum dedicated to it. Start out by doing some reading on communicating between a PC and Arduino.

Lua. or to be more precise LOVE2D

I googled it but didnt find any good information (partially because i didnt know what I was looking for). But im gonna look inside the forum section!

You must have horrible google skills. What did you google? There is a TON of stuff out there on interfacing a PC with an Arduino. There may not be something for that specific language but you can certainly get the idea and then google your chosen language and learn how to use the serial port.

I would rate my google skills as pretty Ok. I just did´nt new the word 'interfacing'. I googled things like "Arduino controll with computer" or "Arduino controll without IDE"

This Python - Arduino demo should give you the general idea. You can do the same thing in any PC programming language.

Also have a look at the examples in Serial Input Basics - simple reliable ways to receive data.