Outlining and visual mapping tools can help you organize ideas before you start writing.
Outlining tools
MS Word has an outlining feature
Google Docs has an outlining feature
Workflowy (free or paid) is a web-based outlining tool with features that enhance focus (expand or collapse) and act as a task-manager (hide completed bullets)
Visual mapping tools
Mindomo is web-based (3 maps free, then paid subscription)
Mindmeister is web-based (3 maps free, then paid subscription)
Inspiration is installed software or web-based (paid download or subscription)