Gmail has automatic spam filtering that filters most of spam straight to the spam folder and you wont see them in your Inbox.
As no spam filter is perfect, you may get occasional spam email to your Inbox. You can mark it as spam and Gmail's spam filters learn from your actions as well and after a while will also learn to automatically mark same type of emails as spam.
- Check the email(s) you want to mark as spam and click on the exclamation icon on the top toolbar
- The email will automatically be moved to the spam folder and will be deleted after 30 days
You can find the same spam icon from the top toolbar, if you've already opened the email!