You may choose to have the catalog display as the front page of your site. To do this, you need to browse to Administer > Site configuration > Site information. The last field on the page is labeled "Default front page" and is the one you need to change. By default this is set to "node," but you can change it to "catalog" to use that URL as the default front page.
(For what it's worth, you can change this to be anything! Feel free to create your own front page some other way, either with a simply page node or using any contributed module, and set its URL here.)