I need some help on finshing some code i cant figure out on making an inventory system?

So I'm making an inventory I have the gui menu for it and the code where the button is clickable and can open the inventory menu but I'm having some trouble on. if someone can give me a couple pointers on how to put an item into the inventory when clicking it this it would be great help. I tried making my own code but I cant seem to get it right. so I don't need the answer to my code but some pointers plz.(meaning don't put the code to do it in the answers I want to try to figure it out but with a bit of help)