Invoices for all orders are generated automatically every 15 days starting from the 1st of every month. You can view all your invoices from Accounts > Invoices.
You can also download a report of all the invoices against which the invoice has been issued. Since all the orders are prepaid you do not have to pay anything for the invoice generated.
The invoice contains all the business details as well as your GSTIN if it is updated under the Profile > Edit Profile section. It is recommended to update your GSTIN as soon as you log in for the first time to avoid confusions in the future.
You can then use this invoice to claim your input tax credit.