How can I create an outline in an entry?

Assuming you're using Tiger or later, simply use the List feature. Show the ruler, if not already, and choose a list style from the Lists menu in the ruler (or the Text > List... menu command, or the List toolbar button). You can then add items to the top level of the outline. To indent an outline item, hit Tab at the start of a line (an existing line or a new line). To outdent a new item, hit Return twice.