What you want to do is to first add an autonav block to your site. This will probably go in header nav, and you want sub pages to display, so make sure to select that. The autonav will probably look messed up. Then, simply add this code to your main css file.

.nav, .nav * { margin: 0; padding: 0; list-style: none;}

.nav { line-height: 1.0;}

.nav ul { position: absolute; top: -999em; width: 150px; /* left offset of submenus need to match (see below) */}

.nav ul li { width: 100%;}

.nav li:hover { visibility: inherit; /* fixes IE7 'sticky bug' */}

.nav li { float: left; position: relative; width: auto;}

.nav a { display: block; position: relative;}

.nav li:hover ul,.nav li.sfHover ul { left: 0; top: 40px; /* match top ul list item height */ z-index: 3;}

ul.nav li:hover li ul,ul.nav li.sfHover li ul { top: -999em;}

ul.nav li li:hover ul,ul.nav li li.sfHover ul { left: 150px; /* match ul width */ top: 0px;}

ul.nav li li:hover li ul,ul.nav li li.sfHover li ul { top: -999em;}

ul.nav li li li:hover ul,ul.nav li li li.sfHover ul { left: 150px; /* match ul width */ top: 0px;}

This code was taken from destyle, which you should also look at if you want to create a dropdown menu.

This will create the dropdown, but it wont style the dropdown, it will be based on the default. If you want to style each part of the dropdown look at destyles superfish.css, its main menu should have what's needed. You may also want to tweak the default.css so that it looks better, I added a black background to the links to make them standout here, I would recommend having a background.