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.
As soon as you have your Categories created you may proceed with your first blog post. This can be done via the Add New button in the Posts tab. You will be redirected to the WYSIWYG editor of WordPress where you can create your posts and categorize them.
If you would like to start your WordPress MU Blog in a few clicks, check the WordPress MU Hosting package.