How to make an inventory?

A simple inventory can be made using a DS List.

/// Create event
inventory_list = ds_list_create();

/// Clean Up event (Always destroy data structures!)
ds_list_destroy(inventory_list);

You can add your items into this list. Let’s say your items are numbered (0 for Apple, 1 for Orange, 2 for Wood, etc.)

You can use Enums to store your item IDs:

enum ITEM {
    APPLE = 0,
    ORANGE = 1,
    WOOD = 2
}

These items (which are really numbers) can be added into the inventory list.

/// Collision event with "oApple"
ds_list_add(inventory_list, ITEM.APPLE);

instance_destroy(other); // Destroy the Apple instance

Similar:

How to draw an inventory