Here's how you can answer, depending on the situation:
Possible answers:
* General answer: "Yo compro lápices, cuadernos, papel, y otras cosas para la escuela." (I buy pencils, notebooks, paper, and other things for school.)
* Specific answer: "Compro un bolígrafo nuevo." (I'm buying a new pen.)
* Negative answer: "No compro nada hoy." (I'm not buying anything today.)
* Question: "¿Qué necesitas?" (What do you need?)
You can also add details about the specific items you buy or why you're going to the stationery store.
Example:
"Yo compro un cuaderno para mi clase de español, porque necesito tomar notas." (I'm buying a notebook for my Spanish class because I need to take notes.)