Technically everything you write is under copyright, as long as you can prove it. This was a problem in the past regarding handwritten or printed works. But on the internet it is easy to prove when content was first published to a website. Hubpages makes it even easier for you by providing the original publication date of a hub in case you forget. So technically you don't have to do anything special to copyright hubs.
There are some authors who simply use the copyright symbol (©) their name, and the year the piece was written. ex. © Chloe VanDuinen 2012 This will deter most humans from stealing your piece (I only use this on my fiction and poetry pieces).
Unfortunately most of the theft occurs from content scrapers, computer programs designed to copy your content and publish it somewhere else under a different name to make someone else money. This is harder to track. I recommend using http://www.copyscape.com to check if your work is published somewhere else, it can also tell you if you're accidentally copying from someone else. Setting up google alerts is another option. You can set it to search google for unique lines from your articles.
If you find that your content has been stolen this article from the Learning Center will tell you what to do. https://hubpageshelp.com/content/Learning-Center-h...