There are basically three ways you could try and protect your designs.
You can trademark the design if it is part of your "trade dress" -- that is, how you identify your company and products. If you are doing a bunch of different custom designs, then trademark would probably not be appropriate -- if everything you design, however, has a distinguishing feature that is common to all of your designs AND is used in your advertising, then trademark may be useful to you.
You can copyright the parts of the jewelry that are "creative" and do not have any "functionality." However, the line between the "creative" parts of a work and the "functional" parts of the work are pretty blurry sometimes, so the level of protection you would get from a copyright may not be sufficient for your case.
Finally, you could obtain design patents on the works. A design patent will give you the most protection, although the protection would be for a more limited time and the process of getting a design patent is the most complex of the three.
You can file for any of these protections yourself. A copyright exists automatically upon creation of the work, but to obtain the most protection from a copyright, it needs to be registered with the U.S. Copyright office. This can be done easily yourself, check out their web page. Although a registered U.S. trademark and a design patent can both be filed for and obtained without the aid of a lawyer, in practice it is not always simple, and is often best left to a pro. A patent agent can obtain a design patent for you for less money than an attorney would charge, but a trademark needs to be obtained through an attorney.
In any event, if you are serious about protecting your work, you should talk with an attorney in your area experienced in patents, trademarks and copyrights to determine the best way to protect your works.