If you own the mark and are applying in your individual capacity and not as a business entity, you may sign the application. If a business entity owns the mark, then the person who may sign the application on behalf of that business is (1) someone with firsthand knowledge of the application contents and authority to act for the owner or (2) someone with legal authority to bind the owner such as a general partner or a corporate officer. If the applicant is represented by an attorney, the attorney may sign the application on behalf of the applicant.
Please note different requirements exist for who may sign other types of documents that applicants must file with the USPTO regarding their application. All documents, including the application, are legal documents; accordingly, the proper person must sign each document for the USPTO to accept it.