Most of your scripts will probably be public instance scripts. Those are scripts used by an instance of a part (class).

Thus, for most scripts, you can keep the push button under the list of categories (left pane) as instance, and the one under the list of scripts (right pane) as public.

If you need to write a script for a class, click on the instance push button so it toggles to class. Then, select (or add) a category, select New Method Template from the Methods menu, and write the script in the lower pane.