How do I get a column's value on a selected row in a form

Hi,

I have what I think is a basic question but I just can't figure out the right way to do it.

I have a form that lists order_number, date, bill_to_name and ship_to_name.  There can be several orders showing in the list.  I want the user to select one (click on it) then click an OPEN button to get more details.  This would function similar to the Oracle EBS Order Organizer, although I'm working with some custom tables.

I have the form displaying the list of orders and I can click on them, but when I click the OPEN button, how do I get the order number on the line that was clicked so I can pull the detail for that order?   What would be the correct way to build something like this?