In order to be secure, a password should be a unique combination of upper case and lower case letters, numbers, and special characters. They should be at least 10 characters in length. Different websites may have different criteria for which characters you can use.
If you want to use a password that is easy to memorize, consider using a long word or phrase that is easy for you to remember but hard for someone to guess.
You can also create a password that is completely random. My favorite method is just hitting a bunch of random keys on my keyboard, and then making sure I have covered all of my criteria (upper case and lower case letters, numbers, and special characters).
Or, if you prefer, you can use a handy tool such as this one from Norton to generate unique passwords.