Formal:
* Soy de [nationality]. (I am from [nationality])
* Mi nacionalidad es [nationality]. (My nationality is [nationality])
Informal:
* Soy [nationality]. (I am [nationality])
* Yo soy [nationality]. (I am [nationality])
Examples:
* De qué nacionalidad eres? (What is your nationality?)
* Soy de Estados Unidos. (I am from the United States.)
* Soy mexicana. (I am Mexican.)
Important: Remember to use the correct gender agreement with "nacionalidad". For example, "Soy mexicana" (I am Mexican) is correct because "mexicana" is feminine.
Let me know if you have any other Spanish questions!