HOW?
Option 1: Google 'CSS 3 column layout'. This is has been well covered over the past 6 years or so and there's gobs of tutorials out there.
Option 2: Google 'CSS Framework' and pick one to build your layout. 960.gs is a popular one.
WHY?
Ideally, you'd use tables for tabular data and css to layout the rest of the page. Why? Well, in theory, CSS gives you a lot more flexibility. The best example is probably when it comes to responsive web design. On an iPhone, I may want 2 columns. On my iPad, I may want 4 columns. That can all be done with CSS, but gets really complicated if you hard-wire the HTML using tables.