Confirm Java script popup in asp.net

I would like to have a javascript pop up messagebox with 'ok' and 'cancel' button when the user clicks submit. also in the message I would like to have multiple lines and not just one line. So the messagebox should look something like

are you sure you want to submit.
document x will be submitted.
document y will be submitted.
you wont be able to revert your changes once submitted.

   ok button      cancel button

can someone please tell me how to have the pop up with multiple lines in the message.