There are actually several things you need to keep in mind when creating and using passwords, but a good password manager will help you take care of the two most important ones. It will make sure all your passwords are long and strong, and it will also make sure each password is used for only one account.
Why you need a password manager
These days, an eight-character password just won't do. Modern password-cracking tools will make short work of it. It's much safer to have a 15-character password.
What password managers do
That's where the password manager comes in. It remembers your passwords for you. All you need to remember is the single "master" password that unlocks the password manager.
The master password should also be a long, but at least it's the only one you'll need to know, and most password managers now offer two-factor authentication to help you secure your accounts with them.
All the stand-alone password managers we've reviewed at Tom's Guide work on Windows, macOS, Android and iOS, and most work on Linux and Chrome OS as well. Each has both a free and a paid version, although most of the free versions limit your account to a single device. (The free versions of LastPass and Zoho Vaultdo not).