Once the sales order is created, view it and click on the "Create Invoice" link in the actions box. This link will only be visible if you have assigned a contact (person) to the sales order. This is so that you can email the invoice to them once it has been created.
Once you have created an invoice you can print it by clicking on the print invoice button, or view/edit it by clicking on the edit button.
If you use Xero for your business accounts (and we highly recommend that you do) you can post the invoice directly to your Xero accounts by clicking on the "Post to Xero" link in the actions box.
Invoices posted to Xero will be in your Xero "Draft" invoices section.