Editing the page is how you do it. Look at this Wikipedia page for an example of how it's usually formatted on other wikis: http://en.wikipedia.org/wiki/Talk:Canada_Day
Basically, you should start a section, where the title of the section is a discussion topic. So if you wanted to ask in the Categorization project how to create a category, you could create a new section like this:
== Creating categories? ==
How do I create a new category?
The section title is sort of like "The question in one sentence" on G2G.
The Project and Project talk namespaces should be editable by anyone (a true wiki). (If anyone has tried to edit one of these pages and hasn't been able to, please let me know!) I think that's the case for the Category namespace as well. Anyone should be able to edit them, not just Supervisors. Otherwise it'll be hard to get everyone using them!