Trademark Application

Probably, a U.S. trademark application on the principal register, on the basis of intent-to-use. See link to the website of the U.S. Patent and Trademark Office.

But there isn't enough information in the question to be sure. Is the clothing going to be sold just in the U.S. or also elsewhere? Is it going to be sold just locally or nationally? Has a search been performed to determine that the proposed brand name (and any logo) are not confusingly similar to trademarks already in use?

It would probably be most efficient for you to consult a local attorney who regularly files and prosecutes trademark applications, in order to develop the most appropriate strategy to protect the intellectual property of your business.

If you already use the trademark for commercializing the products, then use in commerce, otherwise intend to use.
You can register with the USPTO or with your state (check your state has trademark registration)

DISCLAIMER: This answer does not constitute legal advice, and should not be relied on. Each question is fact specific and requires a comprehensive evaluation and consideration of all the facts and documents at issue. This answer does not create an attorney-client relationship.