Joomla has a great feature called a "module". There are actually about 25 different modules that are setup to be displayed where you wish throughout the site.

The most common modules use are:

1) left - used for the left side menu
2) banner - typically found at the top right or very top above the header, used for advertising or graphic banners rotating and trackable.
3) bottom - used to display copyright info, additional menus, and perhaps some cloud tags.
4) debug - THIS MUST REMAIN IN THE TEMPLATE! This is for debugging only and does not affect the layout of the site. However, if you really want something at the very, VERY bottom of the site then use this position.
5) header - used for the logo, site title, or important graphic position.
6) user1 - this is almost always right above the body of your page. the main content. It's used for promotions, specials, member logins, banners, and more. Typically you see this module used only on the home page and select interior pages but not on the content or static pages.