Brainstorming is a technique of listing as many ideas as possible about your writing topic. The greatest rule of brainstorming is to keep the process as broad and open as possible. This video suggests several things that you DON’T want to do.
Mindmapping is similar to brainstorming, but it is much more visual. It allows you to create connections between ideas. It can be a useful step after brainstorming, or it may match your style better if brainstorming seems too random. This video provides a good overview:
Freewriting is a process of simply writing. It helps you get started and can expand your thinking. Watch this video to learn more about this technique: