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.
The client's contact information: Now you'll want to specify who the invoice is being made out to. Include the recipient's name, address, phone number and any other info below the invoice header. Generally, your contact info should be on the opposite side of the recipient's info. So if your business name and logo are on the right side of the invoice at the top, the client's name and info should be below this on the left side.