For most applicants filing in their home jurisdiction, no supporting documents are required. The application itself contains all the necessary information: the mark, the owner's legal details, the goods and services list, and the applicable filing fee.
Foreign applicants may be required to provide proof of identity or company incorporation in some jurisdictions, and are often required to appoint a local legal representative. If a legal representative is filing on your behalf, a Power of Attorney authorizing them to act will typically be required; this is prepared by the representative and sent to you for signature before filing.