The outline gives an overview of how you plan to attack the topic, from beginning to end. One mistake commonly made is to try to get the outline perfect the first time. This usually leads to writer's block. It is generally better to work fast, writing down all the issues that come to you. Later you can eliminate those that are not needed.
Some people find the automatic outlining features in word processors to facilitate the process.
If you forget an issue in the middle, you can add it and the word processor takes care of renumbering. While every outline is different, they generally have several features in common. They start with an introduction that discusses the motivation for picking the topic, the proposition, and other issues that the reader needs to understand the topic and why it is important. They end with conclusions and recommendations. While often combined, these are slightly different.