The only requirement is that passwords be at least 12 characters long. However, you should also take steps to make your password secure.
Tips for creating strong and memorable passwords
Industry standards have changed recently. Experts now recommend:
Good advice is to string a few random words together that you can remember with a visual. A popular comic gives "correcthorsebatterystaple" as an example – memorable with the right visual, almost impossible to crack.
You might also consider using a password manager (e.g. LastPass, 1Password) that generates secure passwords and stores them online.