Regarding password security and management, most accounts aren’t compromised by a hack into individual user computers. Rather email addresses and passwords are stolen in the massive breaches happening regularly among big companies and services, then shared online. There’s little you can do to prevent this, but you can take steps to limit your risk and exposure.
The first line of defense is not reusing passwords. If a password is unique to each specific account, then if one is compromised you limit your exposure to just that one service. If it’s used for many sites and accounts, hackers and criminals can try the stolen email/password combination on other sites and the breach gets worse.
To keep track of multiple passwords, one way is to save them to a protected file on your computer. Don’t use a file named “my passwords” saved on the desktop, rather use a generic filename (like “homestuff”) and password protect the document. Word, Excel, Pages and Numbers on the Mac all support creating password protected files.
For people with many passwords and devices, where a single document becomes unmanageable, use a password manager. This is an encrypted database which can be synced between multiple devices. 1Password is my current personal choice, available in both personal and business versions.