You need a system, not a tool. Once you have a system, you can find the tool (or more likely, small set of tools) that help you best implement it. My system is

You need a system, not a tool. Once you have a system, you can find the tool (or more likely, small set of tools) that help you best implement it. My system is loosely based on Getting Things Done (GTD). At different times over the last 15 years I’ve implemented it using pen and paper; dedicated apps; simple to do list apps; a word processor; etc. Find a couple systems and try them out. Then adopt the principles of each that you find most useful.