Summarize:

Passwords are case-sensitive. Typing "password" isn't the same as typing "PASSWORD". It's located in the upper right corner of your browser. It's located in the sidebar on the left. Here, you'll be able to see any sign-in activity in the past 28 days. Click the back button (left arrow) in the top left corner of your browser next to the URL address bar. If you see any strange sign-in activity or devices you don't recognize, click "Secure your account" at the top of the page.
Log into your Gmail account. Click your avatar. Click "My Account". Click on "Sign-in & security". Click "Device activity & notifications". Click "Review events" under "Recent security events". Go back. Click "Review devices" under "Recently used devices". Secure your account.