To make sure you have a strong passphrase, don't chose common phrases - "To be or not to be, that is the question" or a line from your favorite song, book or movie will not be as secure as a random set of words.
So think about your world and use it to create a passphrase. Someone who's favorite movie is the The Lorax, who grew up in Castle Street as a child, hates spinach and who's sister's birthday is the 12th of April might chose the following passphrase; LoraxCastle12AprilSpinach.
When creating a passphrase to use as your password remember:
Use different passwords for different systems
Don't share your passwords
Make sure you log-off after logging into a system or service
Use two-factor authentication, if available
Don't write down your passphrase if at all possible (consider using a password manager)