First of all, you need to know how to conjugate the verb ser, and here is how to do it: yo soy, tu eres, el/ella/usted es, nosotros somos, vosotros sois, ellos/ellas/ustedes son. You question is Eres alto?, which means Are you tall? In English, you would answer - I am tall. The same thing goes for Spanish - the correct answer is Soy alto, which means I am tall.
