Version Info this help item shows Joomla v1.0 - almost identical in v1.5

Before you write any content items you need to create sections and categories to store the content items in. These can be renamed or deleted as needed.

If we created your Joomla website, your sections and categories are already setup.

How To Create a Section
On the main Joomla menu (right below the Joomla logo), go to Content > Section Manager.

Click the big "New" button at the top of the screen (on the right side - immediately below the main Joomla menu).

Name Your New Section
In the New Section window, enter the new section name in both the "Title" and "Section Name" boxes. Click the big "Save" button.

How To Create a Category
Creating Categories is very similar to creating sections. On the main Joomla menu, go to Content > Category Manager. Click the big "New" button at the top of the screen. Then name your category in both the "Title" and "Section Name" boxes. Click the big "Save" button.