How to use value member with a databound combobox

I have a combobox that is bound to a table. The display member is the column that has the persons name. The value member is linked to the column that has the persons social security number. I have two questions:

What is the best event to use when dealing with comboboxes. I just want the basic one where when the user selects an item, it will run code from the event.

Lets say I have a variable 'x'. How do I set x to the persons social security number on the event when someone selects a name.