You'll find everything you need in this area of the help centre to get started using invoicing.
To get started, here's a quick demo of how to send an invoice from start to finish:
Just so you know, if you don't have all the information you need to create your invoice when you start making it, you can save it as a draft at any time and come back to it later.
Just push "Save as Draft" in the bottom left while you're walking through the steps to create an invoice.
You can also find more information on how to:
Manage your invoices (see which invoices you've got paid, unpaid, late or unsent)
Automatic matching of your invoice payments (and how to mark invoices as paid)