First look up "portal" in FileMaker help. It's got enough details that I'd prefer you read that entry before I try to describe it fully.
Then learn how to define relationships between tables as you need such a relationship in order to get the portal to work.
Then take a look at this simple invoicing demo file that uses the portal for data entry but which does not use it for printing the invoice: http://fmforums.com/forum/showpost.php?post/309136/
And after that, feel free to post follow up questions in Post A Answer below if that is not enough to get you going.