question about how to operate pop-up window..

 this is my design


when page _load pop-up a winodw which is a list

I want to ask user to select one id from pop-up window
if user choose one id from list, I will pass the value to main page

if user doesn't choose any id from list and  close windows ( to press the X to close pop-up window)
I want to show a alert say main page will be closed because you don't choose any id from list


question is


how to pop-up window when page_load? ( do I need to write javascript? or could be down by server code?)
how do I know if user click "X" to close window ?
how to pass value from pop-up window to main page?
I think the code just like this


page_load
{
//step 1 pop-up window
//step 2 make sure if user choose one of id from pop-up window
//step 3 if user choose id from list pass value back to main page , if use doesn ...