how to make dropdown menus in sidebar

hi guys,
i want to make dropdown menus in sidebar. I use custom menu.
this is the link pandjalu

you will see, green and black menu
green is main menu
black is submenu

what i want is when i click the green… then the submenu (black) will dropdown -and stop

and when I click another main menu, the first dropdown menu will gone with the new dropdown of second main menu/another menu…

if what i want is really imposible, how to make the dropdown using mouse hover..

thanks in advance