I don't exactly know how CSS works with respect to allocating of "table-like" boxes for content in my html code.

http://www.two-way-street.blogspot.com

So here's my question. How do I make opacity in the places where I type the content and the side menu bar? How do I change the colour and of the background in that area and opacify it?

Do make something transparent in CSS you simply put:
filter:alpha(opacity=50);-moz-opacity:.50;opacity:.50;
This will work in most, if not all, browsers. 50 is half transparent.

Source:
http://www.mandarindesign.com/opacity.html

Hope I could help!
~WebBaka

Hmm, but that makes the text in the area opacified too. How do I add a background colour to the area where the content is and opacify just that background?

The only way that I know of to do it using the opacity attribute is to have one div that's just the background and make that translucent, and then position another div over it with no specified background color and just the text, so the background appears to be translucent, while still having the text at 100%. :)