I want to build a "second brain" to organize my thoughts and ideas. What's a simple tool for that?

We ask the same underlying question in different ways.

• I want to build a "second brain" to organize my thoughts and ideas. What's a simple tool for that?
• I need to organize my notes and ideas. How can I create a personal knowledge management system?
• Can you guide me through the process of creating a personal knowledge management system?