The USPTO requires a couple of necessary documents before filing a trademark application. Firstly, information about the future owner of the trademark must be stated. In the case of the legal entity (company), it is the company name and the official company address. If the owner is a physical person, it is the name and the official address of the person in question.
Secondly, if you are applying yourself, your signature is required. If you are using a law firm, an attorney or an online legal service such as Trama, they will represent you before the USPTO. You will be required to sign a Power of Attorney so that your representative can act on your behalf.
Moreover, the filing basis (either 'Use in Commerce' or 'Intent to Use') must be stated together with detailed information about the trademark. For use-in-commerce applications, you'll also be required to submit proof of commercial use of the trademark, a so-called specimen, so you can start gathering it in advance.
Lastly, you have to include the logo or the wordmark you wish to register. It is a common practice to provide a logo in black and white. This way, all colour variations are protected upon successful registration.