The first thing you'll want to do is create a new lead, right from your account dashboard's New Lead button. A new lead is a job that isn't yet booked. You'll add the contact information of the person who will receive the invoice. After that, go to the Quotes & Orders tab where you'll recreate a new order with the items they're purchasing from the Products you have set up in Settings.
When you save and book that order, it will create one or more open invoices, based on the assigned payment schedule, that you can then email to the client. You can read more about that process here: