The United States does not have a granting authority, but you may wish to have arms granted for your use by the College of Arms in the U.K., the Lord Lyon of Scotland, the Chief Herald of Ireland or other Heraldic Authorities.
You can also have your ancestor's arms properly matriculated by contacting the granting authority in the country in which the arms were originally granted. This is the proper manner to use ancestor's arms.
You may wish to work with an experienced heraldic artist who can help you to divise original arms for you to assume. In this case, for Americans, the best protection for those arms is to have them listed in one or more heraldic registers (thus providing at least some indication of their first use).