A good password should have a minimum password length of Eight (8) characters, however you should note that longer passwords are generally more secure.
the use of both upper-case and lower-case letters
inclusion of one or more numerical digits
inclusion of special characters, such as @, #, $
do not use words found in the user's personal information
do not use a company name or people's names
do not include: Ambiguous Characters eg: ( { } [ ] ( ) / \ ' " ` ~ , ; : . < > )
do not use passwords that match the format of calendar dates, license plate numbers, telephone numbers, or other common numbers
If you want to use a RANDOM PASSWORD GENERATOR try this one: http://passwordsgenerator.net/
If you want to read more about this subject follow this link https://en.wikipedia.org/wiki/Password_policy
If you want to know more about how to set up a strong password click here