Needing some help with this research as to what are the steps to become a U.S. Citizen? and what documents are needed to apply?

Here are the facts on this case.

Mr. Wang is from Canada he was attending school in Colorado. He has since received his PhD. He's now married to a U.S. Citizen and they together have 4 kids all born in the U.S. He's working for the university on cancer research. He has been lawfully present in the United States from graduate school into his current status on an employment-based visa.