If you want to use Bugzilla, first you need to create an account. Consult with the administrator responsible for your installation of Bugzilla for the URL you should use to access it. If you're test-driving Bugzilla, use this URL: http://landfill.bugzilla.org/bugzilla-4.2-branch/.
On the home page index.cgi, click the "Open a new Bugzilla account" link, or the "New Account" link available in the footer of pages. Now enter your email address, then click the "Send" button.
If none of these links is available, this means that the administrator of the installation has disabled self-registration. This means that only an administrator can create accounts for other users. One reason could be that this installation is private.
Also, if only some users are allowed to create an account on the installation, you may see these links but your registration may fail if your email address doesn't match the ones accepted by the installation. This is another way to restrict who can access and edit bugs in this installation.
Within moments, and if your registration is accepted, you should receive an email to the address you provided, which contains your login name (generally the same as the email address), and two URLs with a token (a random string generated by the installation) to confirm, respectively cancel, your registration. This is a way to prevent users from abusing the generation of user accounts, for instance by entering inexistent email addresses, or email addresses which do not belong to them.
By default, you have 3 days to confirm your registration. Past this timeframe, the token is invalidated and the registration is automatically canceled. You can also cancel this registration sooner by using the appropriate URL in the email you got.
If you confirm your registration, Bugzilla will ask you your real name (optional, but recommended) and your password, which must be between 3 and 16 characters long.
Now all you need to do is to click the "Log In" link in the footer at the bottom of the page in your browser, enter your email address and password you just chose into the login form, and click the "Log in" button.