There should be a table for Clients (if you have more than one) and a table for payments (if you have more than one per client)
Clients each have a unique ID number. The Payments each have an ID number that matches a client ID. That is the relationship.
A portal created on a layout based on Clients Table and based on the ID number relationship will only show the payments of that client.
The first 5 links listed below are the best source of basic guide to Filemaker I am aware of.