First, make sure you're signed into Google, and then go to Gmail.com or the Google homepage. Next, click on your profile picture in the top-right corner of the webpage.
Next, check if there's a link for "Google+ Profile" beneath your name and email address in that window. If the link shows up, you have a Google+ account. Click the link to be taken to your Google+ profile.
Once you're at your Google+ profile, click on "Settings" on the left side of the page.
Scroll to the bottom of the Settings page, and click "Delete your Google+ Profile." On the next screen, you'll be prompted to confirm that you want to delete your account.
Once you confirm that you want to delete your account, you'll be greeted with this survey page — that means you're all set, and your account has been deleted.