Javascript for improving

I have experience of some different programing languages. Also Javascript, I've learnt some ideas of it. But it is 6 years ago...ages..
I'm thinking that I'm gonna use some Javascript to improve my minisite 1 to minisite 2. It may be the part of layout, menu, or rollover images, etc. But I know that there alread exists rollover scripting in Dreamweaver and other html programs. Itsn't there anything else that I can use it for my site?

As my site structure has both Main menu and Sub menu, I may use rollover javascript. With div and javascript, I can make rollover menu layout. When user rollover on main menus, sub menu automatically appears under the main menu bar, or something.

I just recall this idea from my previous work, but not sure how I can achieve this effect...I need some research.

Still struggling with new design of minisite2. menu will be javascripted. what about texts? and images?