The Trademark Trial and Appeal Board (TTAB) is the administrative body that decides trademark oppositions and trademark cancellations. A trademark opposition is appropriate when a third party wishes to oppose the registration of the a pending trademark application. A trademark cancellation is appropriate when a third party wishes to petition to cancel a registered trademark. Thus, one must first determine whether the trademark is simply a pending application or a registered trademark.
To determine whether a trademark is pending or registered, you can access the trademark record via the Trademark Electronic Search System (TESS) of the United States Patent and Trademark Office (USPTO). A pending trademark application will only have a Serial Number. A registered trademark will have both a Serial Number and a Registration Number. Moreover, a registered trademark will have a Registration Date.
Thus, once you have determined that you need to do something about this other trademark, it will be imperative for you to be able to identify whether you can file an opposition or a cancellation proceeding. A trademark attorney can help you navigate both a trademark opposition and a trademark cancellation.