It depends. If someone say "잘지냈어(요)?" which means "have you been well?" then you can say "응/네, 잘지냈어(요)" which means "yes I have been well". If someone ask you how you are feeling you can say "나는/저는 괜찮아(요)" which means I am Okay or fine.

나는\나\난\저는\내가 잘 지내요 ????

괜찮습니다, 괜찮아요, 좋아요