1. Click the Project tab (not the one at the top of the Construct window), then look at the list it gives you. You should see a folder named "Global Variables".
2. Right-click on that folder and click on "add global variable".
3. A small window will pop up named "Construct: Add global variable", and this is where you will name your Global Variable (in the first field), decide whether the Global Variable type will be text or a number (see the pull-down menu), and decide what the initial value of the Global Variable will be (third field).
4. When you've chosen all of what you want to do in #3, click OK, and you've created a Global Variable.
Now the Global Variable is in and can be used in events.