How can I find out which template to edit?
Using your browser, view the page source and look for a line of code near the top of the page which begins <div id="content" class="; the class is the name of the template. So for the main forum page, the line of code is: <div id="content" class="forum_list">, which makes the template forum_list.
I am very happy with my sidebar. It's now on sidebar_online_users and category_view (dedicated cat page for each category)
I added a table between the code brackets that create the sidebar. This displays links to my main areas to act as a contents menu.
The table is in old fashioned html as I'm not any good at css. It looks OK at least for now.
But the little graphics for the categories don't display.