Before jumping into the game view you'll have to first define at least one item. Items, Recipes and everything related to the Inventory is located at the Game Creator's preferences menu. To open it, simply click on the toolbar's Game Creator button and select Preferences.
You can also open the Preferences Window using the keyboard shortcut Shift + Ctrl + K if you're on a Windows machine or Shift + ⌘ + K in macOS.
Navigate to the Inventory left-sidebar button and you'll be presented with a screen with 4 options:
Catalogue: Here you can create the game Items and give them properties
Recipes: In this tab you can specify what happens when combining two items.
Types: Create item types for better organization and equippable items.
Settings: Everything related to the Inventory's settings is here.