Copyright protection is automatic when you create a tangible artwork and it is already considered your intellectual property. This means that you retain all the rights to your work, so no one else is allowed to reproduce, share, publish, or profit from the art without your consent.
If you work in traditional mediums, one important way to copyright your art and mark it as your own is by signing your finished artwork. To prevent people from stealing the images of your work posted in your online portfolio, you should probably be signing the front of your work as well as the back, so that shared images will still have that information on them, even if you haven’t been credited. The more information you include with or on your artwork, the easier it is to determine ownership should you need to prove that your artwork has been used without your consent. In the US, you can copyright artwork by registering it through the Library of Congress Copyright Office.
If someone is attempting to pass your creation off as their own, keeping extensive, detailed digital records and photographs of the artwork in question will be a huge help in proving ownership. Digital documentation is great too because it will often contain relevant metadata which can prove exactly when the photograph or document was created, which can be key in proving who created the image in question first.