none will tell you here step by step how to do a full inventory, that's way too much work (which is why there is one in the store for 40 dollar). But we can give you advices on how to start it. Using arrays alone can be enough for a simple inventory system btw.
But here is how I would do it:
Use a 2D-Array. (X = ID of item, Y = is item slot free) When you drag and drop item X into the inventory you check for an empty spot in the inventory. After you set the ID of the droped item into the X axis.
If you want to save multiple items you could use an instance variable of an object to save its amount. (If you drag and drop an apple onto another apple in the inventory you add to the apples instance variable ("amount") +1.
What did you try so far? Did you try something on your own without any tutorials?