आप कैसे हैं ।- with due respect to someone.I would not say formally because respect can be given to younger also.
तुम कैसे हो । - how are you. ( to someone who is younger and to some1 of same age)
तू कैसा है - to some one who is very familiar like friend.
कैसा is used for masculine gender. कैसी is used for feminine gender.
Correct me if iam wrong or elese u know wat do - upvote the comment.