How do I set a boolean variable to switch between true or false.
Example: I press g and variable gets set to true. Press it again it returns to false
Since this is very basic, I recommend this :
These are really great tutorial. For your actual question, here is a blueprint that will do exactly as you asked:
Only the first part is needed, (the SET), after that it’s only to print the result on the screen.
Hopes this sets you on the right path to learning UE4 
Mick
How do I get the thing with the NOT letters?? And thx btw
Just right-click and type “not”. Since you want to do that on a boolean, the correct one will be under Math → Boolean and will be called “NOT Boolean”