A strong password is a sentence that is at least 12 characters long. Use sentences or phrases that you like to think about (for example, “I love soft cookies.”). On many sites, you can even use spaces!
It can be hard trying to remember a unique password for every account you have. Luckily there are purpose-built password managers that can take care of the memorization for you. Two of the most popular currently are 1Password and LastPass. Adding two-factor authentication will make your logins the most secure. You can read a how-to for adding two-factor authentication for online sites at TurnOn2FA.com.