For now, let's try making a new user and a new blog.
If you're not already logged out, click Log Out on the front page of the site.
Enter a username and an email address. Leave Gimme a blog! ticked, and click Next.
Choose the subdomain you want, give your blog a title, and click Sign Up.
If everything goes well, you should get an email within a few minutes, and clicking the link in the email should send you to a confirmation page showing your username and password.
Tip
If your signup didn't work
If you don't receive the signup confirmation email, check the email settings in your WordPress MU control panel. If you get the email, but the link doesn't work, then the problem lies with the subdomains setting. A good place to look for advice when it comes to setting up WordPress MU is the official forums, which can be found at http://mu.wordpress.org/forums.