If your LLC is a single-member LLC, then the single member will, by default, be the only authorized signatory. If the LLC is a multi-member LLC, then the managing member is usually the one with authority to sign, but it depends on the LLC’s governing documents.
An LLC’s operating agreement can specify who the authorized signor is, whether that be a manager, member, or third-party to whom the members agree to designate signing authority. If the LLC does not have an operating agreement, then a resolution signed by all members of the LLC authorizing an individual to sign on behalf of the LLC can be sufficient.