From Keep your account secure - Google Accounts Help
Make your Google Account more secure
1. Check for viruses and malware.
Run a scan on your computer with a trusted anti-virus software. If the
scan detects any suspicious programs or applications, remove them
immediately. Read
more.
2. Do an account Security Checkup.
Go to the My Account page. In the
"Security Checkup" section, select Get Started and follow the
on-screen instructions to check your account security.
3. Update your account recovery options.
Check that your account recovery options are up-to-date.
4. Enroll in 2-step verification.
2-step verification
adds an extra layer of security to your account by requiring you to
sign in with something you know (your password) and something you have
(a code sent to your phone).
5. Perform regular operating system and browser updates.
Make sure you've set your computer to update automatically. If you get
a notice to update, take care of it right away.
6. Never use your Google Account password on another website.
If you enter your password in an external website and it's
compromised, someone could try to sign in to your Google Account with
the same information. Make sure to change your Google Account
password.
7. Protect your password.
Never enter your password after following a link in an email from an
untrusted site. Always go directly to mail.google.com or
accounts.google.com/Login.
8. Use a strong password.
Don't write it down, send it via email, or tell anyone. Google will
never email you to ask for your password or other sensitive
information.
If you use a public computer, take these extra steps:
Always sign out of your account. Just click your name or email address
at the top of the page, and select Sign out. Clear forms, passwords,
cache, and cookies in your browser on a regular basis.
Note: You should perform these steps more frequently if you begin to notice suspicious behavior in your computer, such as general
slowness and pop-up advertisements. For more tips about online safety,
visit Google's Safety Center.