Using levels as a menu system

Hi all,

I have a game nearly ready to release. Rather than using the built in GUI functions, I'd rather use the level editor to create my menu system.

After the splash screen, I want to display the main menu. On the main menu there will be a few basic options:

-about
-how-to
-play vs
-play computer

clicking on any of these options should close the existing level (menu), kill any objects used to create it, and launch a new level which could be informational, or launch the game.

There are three things I'm not clear on:

1) What determines the first level loaded, when the game launches? Where can I configure this/hard code it?

2) How do I associate scripts with levels, or are they all interchangeable?

3) How do I close/end/destroy an old, unused level and all associated objects if it's no longer needed?