Depends on the question. If it's the normal English greeting "How are you?" we usually answer with one word unless we're being formal: "fine" and "good" are both acceptable, but "good" is more positive.
In very proper English, you would say "I'm well," instead of "I'm good." (Some people will insist that "I'm good" means "I'm a nice person.") But at least in the USA, it's much more common to say "I'm good."