For any type of business to succeed, you have to get paid. So whether you do one freelance gig a month or run a full-time writing business, you need to calculate the time you've worked and bill your clients for it. That means you need to know how to write an invoice.
A professional header: The first item on your freelance invoice should be your business name, or your full name, in professional (and easy to read) font. The font size should be a little bigger than the rest of the text on the invoice, and possibly even bolded for emphasis. Next you'll want to include your contact information: your mailing address, phone number, email address, website, etc., right underneath your business name.