Hi,
That’s a tricky question, even though it seems simple. I’ve used Suckerfish for this and various other canned dropdown plugins. But they all use quite a bit of markup, complicated CSS, and may also have JQuery and such for further effects. Not only that, but due to extreme complexity, they often don’t survive WP upgrades. One really nice one that I used to use simply does not work in WP3 at all.
Here’s what I would do, and I think it’s too vague an answer for you. What I would do is find a theme that has its own dropdown built-in, and tweak that. A whole lot of themes are prepped for dropdowns and work nicely. When you’re demo-ing themes under Extend…Themes, you can usually see pretty quick if the dropdowns work well. If you find a theme with homegrown dropdown CSS, you have at least a chance of having clean code and no fuss.
Beyond that, I would make my own theme and code the CSS for the dropdowns myself. Then I can be assured that it will work. But that’s not a newbie task by any means.
Maybe someone else will have an easy answer.
Good luck, Dave