Your inventory contains all the pieces of work you hold in your gallery. In order to add a new piece to the inventory:
Go to Inventory and click on the New button on the menu. You will be taken to a screen with several tabs to work through. Click on the save button regularly to ensure your work is saved.
As a minimum your inventory record needs to have the Title and the Artist. See Adding, editing and deleting for more information on completing the rest of the record.