It is important to use secure passwords for your various online accounts and to use different ones for different services that you use.
The following resources contain useful advice for selecting secure passwords that you will be able to remember:
Cyber Aware Advice
Three word style: https://www.cyberaware.gov.uk/passwords
Other Password Advice
If you want something really secure, follow this advice: SophosLabs
If you want an animated lecture about password good practice and common mistakes click here