In Maintenance > Order & Items > Item Types, you can add a new item type by clicking on the button, this will open a new window.
In this window you can define the name of the item type, the product type it will apply too, how to charge for the item type, how to invoice and any additional options. You can also add a unit description and an initial stock level. Finally you can add an issue quantity and select a default nominal code. Click the button to save your changes.