Need help with CSS drop-down menu issues?

I've created a CSS drop down menu for my site's navigation...but I'm having an issue I can't figure out. When the menu drops down, it shows up behind the content on the page. The writing is showing through the menu, as is a picture (which makes the menu impossible to read). The site is set up with tables. I've tried layered tables and iframes, and nothing seems to solve the problem. I'm new to CSS....how do I fix it?? I don't want to have to move my content

Remeber one thing that the z-index property only works on positioned elements.
So make sure your drop-down menu defines its position property as either fixed, absolute or relative.
Then, define the menu's z-index property as a high number. You may also need to declare the position and z-index properties of the other page elements (setting their z-index property to a lower number).