Main rule: How secret is the thing I want to protect with the password?
Use Two-Factor-Authentication wherever possible.
The longer and more different characters, the better.
Choose characters that do not form words to protect against dictionary attacks.
Choose something that is not related to you to protect against social engineering attacks.
Choose different passwords for different accounts.
Use a trusted password manager for convenience.
Choose a strong password for your e-mail account, because almost all other passwords can be reset by e-mail.
My Personal Tips
Use the first letters of your favourite song text or movie quote.
The quote should contain special characters in the best case.
Use words you spelled wrong as a child.
Youtuber’s and Twitch streamer’s names are often a good pick as they often contain
special characters, are easy to remember and the words are uncommonly found in a dictionary.
Just don’t choose someone too popular
Use an adress (state + zip code, street + house number for extra security) that is not related to you
If you want to use similar passwords for different accounts, use a hashing method.
