Auto Filling from the same field.

Auto Filling from the same field.

Hi!
I have an application that can be used by both new clients and existing ones. The form has a decision box which when checked distinguishes the user as an already existing client.
When this is checked a look up table is shown with all the existing clients, Once a client is selected I want to be able to auto fill his client information, Email, number, etc..
I am unsure how to auto fill this information when the information I need comes from this forms report.
Thanks!