how to add logic within a field

Reuben:   Rob thought you might be able to help.

I am new to Zoho...and am trying to understand a couple of things.

I am creating a form. I want to add logic to a drop down box....or a check box.

The first check box choice:

Add a new customer in the data base.
OR
Edit an existing customer record

If they select the "add a new entry"....the form should then display the fields they need to complete for a new customer.

If they select the "edit an existing customer"     they should see a list of existing customers by name.....the user can then click a name and edit fields stored in the data base.

???   What type of logic do I need to make this happen?
???   Do you need to use the Reports feature to create a list of existing customers?

thanks

kathy