In GTD David allen said, “Your mind is for having ideas, not holding them.” It better to write down everything. what ever in comes in mind.
The problem is we forget things. Because a lot of things going a round us. Our focus changes from point to point. Different people need our attention. And what Idea comes. What important task we need to complete, we forget. So You need simple diary. Pieces of paper or some note taking app on computer. Note down everything. It will help you to remember things. later you can add more information on these noted thing. Add and remove things. Which completed, which need to complete or no need to work on.