Get the Mission Editor, start the game with it, press F10. You may find it easier to copy the contents of something like the admin menu GUI so you have a list and scroll box, press File>New (name the GUI and leave the "GUIControl" class part) and then paste it so you have things to work with. Change the close command of the window and cancel buttons if you make them to the name of your GUI, then change the names of the list. Look at people's scripts for things with lists (Team Deathmatch, Macro Saver, JVS Portals) for how to make lists work.