GUI button

Hello,
I Wanna create a GUI to increase the satuts of my character, like damage, life, defence, etc..., but Idk how I insert functions to my buttons, and how could I make the exp part, I would like to be separete from the exp to enchant things.

And how do I change the size of the button? I've tried sometimes and some different ways to do it, and I just can't do it.

Thanks for responding