A different password for every different account would be the best way to secure your accounts. The obvious problem – how to memorize all the different complex passwords?
Password managers store all your passwords securely in one place. That would mean that you only need to know the password to your password manager.
You could write them in a notebook. The only problem – you could lose it or a person could get their hands on it.
You could store your passwords in a dedicated txt. file on your computer.
Warning: Some viruses search for such files.