Populating an application item variable.

I have an application variable declared in Page 0 that I need to populate when a user first logs in.  Once they get to the home page, I want a pop-up window to open that displays a drop down list.  The user selects an item from the list and the application variable is populated with that selection.  I have two problems.  One is how do I populate the application variable?  I am assuming I need to populate the variable through PL/SQL so that it is stored through the database when the page is submitted.  Also, I only want the application variable