The question, ¿Cómo eres?, does ask "How are you?", but uses the verb ser, meaning "What are your qualities?" instead of the verb, estar, meaning "How are you NOW?" Because of this, Estoy nervioso and Estoy enfermo are incorrect answers. "Eres" is the tu form of ser, meaning that someone is asking the question to you. You then must reply in the yo form, and "Soy" is the yo form, wheras "es" is the el/ella/usted form. Soy alto, meaning "I'm tall" is correct.
