When you want to add a new issue, first make sure it's not already posted in some form by using the search facility!
If you have any issues, press 'Create New' and enter a short, descriptive title, the priority (an indication of how urgent the problem is), set the status to unread, make sure you add some names to the nosy field (everyone in this list will get an email for each post on the issue, if you don't enter any names here it may take a long time before anyone notices your issue), guido_w is generally a good one to add, and add a note in the 'Change Note' field. Superseder should only be set when an issue is superseded by a new one (so it's never set on a new issue), assigned to will generally be set by the person who decides to tackle the problem, and topics is a field you *may* want to fill to categorize the issue, when unsure leave it empty.