To start, Login to your Google account here: Webmaster Central
This login is the same as your Gmail account, or YouTube account logins.
Once you have logged in, you want to click on "Add a Property"
From here, click on "Alternate Methods"
From here, you will want to click "HTML tag" and copy the code it presents as illustrated in the below with Red Arrows.
NOTE: Leave this page up, you will need it again.
Now you will need to login to your Dashboard on your website. From the Dashboard, click "Settings".
Now, Scroll down to the section that says "HTML <head> Code: Advanced users only" and paste in the code you copied from Webmaster Central on Google. Once you have pasted the code, click "Change <head> HTML Code".
Now, go back to the Webmaster Central page on google, that you should have left up from the previous steps. If you accidentally closed it, you can find it again here: Webmaster Central
Click the "Verify" Button at the bottom of the page. This will give Google the command to check your code, and verify the site.
If you did it correctly, you will be presented with a page that looks like this: