Go to http://wiki.ucl.ac.uk and login with your normal UCL user ID and password.
The page which appears is the UCL Wiki Dashboard. It lists the spaces (i.e. different areas in the wiki) to which you have access in a column on the left. You may be able to see some spaces without logging in, but you won't be able to do much.
How do I request a new UCL Wiki Space?
UCL Staff can request a new wiki space using the Confluence Wiki Space Request form.
How do I add a comment to a page?
As you navigate through pages in spaces, you will find Add Comment links on some pages. If you have a short comment about the page, you can use these. Someone else may reply to your comment, thus creating a threaded discussion. If your comment is more substantial, you may want to edit the page itself, if you have been given the right to do so (you can see the Edit tab at the top of the page).