Creating an inventory is not an easy task, it may be a bit too advanced for a beginner. Especially if you want multiple npcs in the game to have their own inventories.
One way to do it is to add an Inventory array (or dictionary). When any npc is created, also create a new instance of this array and link it to the npc using UID or some custom npc ID.
There are lots of other things you need to consider when designing an inventory - how it will be displayed, how items will be added, stacking/non-stacking items, limiting stack size, combining/splitting stacks, using/dropping items etc.
I would suggest to start by reading tutorials, there are quite a few:
construct.net/en/tutorials/search