Be advised that WordPress 2.9 MU is no longer a separate project from the main WordPress branch. It is continuing development as part of the main WordPress 3 branch under the name ‘multi-site’ or MS. This means that you can get all the benefits of both WordPress and WordPress MU versions by using the latest WordPress version. To help you in the process of creating a multi-user website with the latest WordPress version we have created a detailed step by step WordPress multi-site tutorial available here.
To start creating your content for your blog you should first create categories for your posts. This can be done via the Admin area -> Categories tab from the menu on the left.
From the Categories page you can add a new category or organize your existing one.
If you would like to start your WordPress MU Blog in a few clicks, check the WordPress MU Hosting package.