Marking an email as spam:
If you get an email in your inbox that is actually spam, you need to click the "Report Spam" button (looks like an exclamation mark inside a stop sign) in the top toolbar to move that message into your spam folder and to help teach Gmail to treat such messages as spam in the future.

Checking your spam folder:
Every few days you should check your spam folder to make sure no important email messages have accidentally been identified as spam. Simply click on the folder titled "Spam" on the left side of your Gmail screen to open your spam folder.

If everything in the spam folder is actually spam, you can leave it alone and Gmail will delete the messages automatically in 30 days. However, if you want to delete them yourself, simply select the message(s) and click the "Delete Forever" button in the top menu bar.

Marking an email as not spam:
If a real email message gets accidentally marked as spam, you can remove it from the spam folder by opening the message and clicking the "Not spam" button in the top menu bar. This will move the email message to your inbox where it will be safe.

https://sites.google.com/a/bbns.org/tech-day/tech-day-e-mail/handling-spam-in-gmail