When applying for a trademark in your domicile, generally, you won't have to submit any documents outside of the details you provide in the application itself. If you aren't a citizen of that particular country, you might be required to submit a form of identification such as an ID or a passport in the case of a physical person or the incorporation certificate in the case of a business.
Additionally, if you are applying through a legal representative, you might be requested to sign a Power of Attorney. This document will grant your representative the right to represent you during the process and allow them to conduct the necessary actions on your behalf.
In the US specifically, if you are applying under the "use in commerce" basis, you will also have to enclose your specimen of use - one for each class. If you are applying under the "intent to use" basis, you will have to submit them later.