How do I get citizenship for my baby who will be born in Mexico to a Mexican mother?

Under most conditions, the child of a U.S. citizen who is born in another country is a U.S. citizen at birth. Locate the nearest U.S. Embassy or Consulkate to where you are living ands ask about the process for obtaining a Consular Report of a Birth Abroad.  There are strict requirements to documents the facts, such as your own citizenship status, and that the child is yours, so follow their instructions carefully.  Good luck.