Please read our Password Policy on IST wiki for full understanding of password safety rules.
A primary factor regarding password strength, is its length. Short passwords are easier to break. The minimum is 8 characters. Do not use one word passwords, like "football", which is very easy to break.
Do not use one password for multiple accounts. If one is hacked, all the others are compromised by that single password.
The password is a secret, do not share it with anyone else and keep it only in your mind (especially, if it is a master password of a password manager).
A strong password is created from randomly chosen elements like random words, random numbers or random letters. For example: do not use phrases from literature and movies (like "be or not to be"), song lyrics, titles, names, dates or anything that other people could possibly know. Do not use sequences generated by simple rules like "123456", "qwerty" or "3.141592".
One of the possible ways to create a strong and relatively easy to remember password is by composing it of several randomly chosen words and additional number(s).
For example, the password "restrictsScuffling93pickaxed!" is composed of: