It must be "I'm from the USA." This is because "USA" is an abbreviation for United States of America, so here we must use the definite article "the" with the noun "states." You can say "I'm from America," I'm from Alabama," "I'm from Berlin," etc., but if the noun is pluralized the definite article is required. "I'm from the Azores," "I'm from the Balkan Islands," etc.