I want to be able to have the user type a number or use the slider bar to pick a value and have that value be stored as a global variable. How can I do that?
For getting input from the user, you can use the TextBox Object in Construct 2. If you put one in your layout, you can access it's current value in the Event Sheet at any time. You can use the On Text Changed (below) event and grab the current value by using the Text property (also below) and assign it to a Global Variable.
TextBox Manual https://www.scirra.com/manual/117/textbox
On text changed Triggered whenever the text in the field is modified, by typing, backspace/delete, cut/paste etc.
Text Get a string containing the text currently entered in to the field.