Answer to Question 1
Brainstorming is simply listing all the ideas, facts, possible leads, and anything else that may be helpful in constructing your message, until you run out of ideas.
Mind mapping may be more graphical, for example, starting by writing the purpose of your message in the middle of the page. Then you add thoughts and draw lines indicating their connection either to the main purpose or to another point.
Answer to Question 2
Organization is the process of determining the order in which to present topics. The first step is to classify or group related ideas together. After grouping related ideas, you should identify major and minor points to get a better sense of how the message will come together. The organization depends on the reaction that you expect from the reader. If you anticipate a positive response, the direct approach may be appropriate. If you anticipate a negative response, a more indirect approach may be preferable.