Hey, you should click: Add event > System > Compare variable. In order to get the variable you should right click at the very top of your event sheet and choose 'Add global variable'.
I could explain it all step by step but I'm thinking if you try the Construct 3 beginners Tutorial, you will learn all of this and much more :) Otherwise it would be much too time consuming for me.
construct.net/en/tutorials/beginners-guide-construct-1
Of course, if you get stuck with something you can always ask here.