You don't know mind mapping? Oh boy it's your lucky day!
Mind mapping is a technique to help you sort out your ideas, and can be used for project planning, todo lists, project management, brainstorming and all sorts of things (I've even seen it used for bugtracking/bugfixing). You just make a big diagram to categorize your ideas/tasks/whatever in a hierarchical form, thus creating a "mind map" of your thoughts and their connections. It's sincerely fantastic for everything (including game design and programming), and vastly superior to todo lists.
Check out freemind for a FOSS alternative
There's also simplemind as a good commercial option. Also available for Android and Apple, and it's really worth the price.