I Want To Make A Score That Changes When You Click A Button And The Score Shows Up In A GUI

Here's an example of how you would return the score, (if it's a player number variable), and how you would set the score using a player number variable.

If you instead want to use NBT data, the first function here will instead increase the score based on the NBT tag of an item in the GUI, and the second function will simply set the NBT tag of the item. (You would need to do this when the player first gets the item, otherwise the tag will default to zero.) 

Yes- the problem you're probably having is that you need a way to save the numbers so that you can refer to them later. Procedures normally just get rid of local variables when they're finished, so you either need to have a player persistent variable, or NBT data that will stay permanently. The 'Return Text' function, (and all the other return functions), allow you to send the result of a procedure to another procedure, GUI, entity, etc, which is useful for conditions, displaying variable information, or if you just want to make a separate procedure to do something for you that would be too clunky to include in the main procedure.

For strings of text, you can use a single text box, or use a 'create text with' function, which is useful if one or more items of the text are a variable. 

...Also, please try to just send one post with questions. I'm happy to help, but I get email notifications, and it's starting to clog up my inbox...

What are you using as the trigger? If you're doing it on entity update tick it might go down rapidly, or if you're accidentally multiplying instead of adding? It's really hard to tell without seeing your code. 

You should be able to pretty easily make a command that resets the counter. If it's a variable, just make a custom command, and link it to a procedure that sets the variable to zero for the target entity.