①元気?(genki)
and
②調子どう?(choushi dou) mean How are you?"
, or
③初めまして(hajimemashite) means "Nice to meet you"
...so,responses are for example...
①元気だよ。そっちはどう?(genki dayo. socchiwa dou) measns "I'm fine,and you?"
or
②あまりよくないな。(amari yokunaina) means "not very."
or
③こちらこそ初めまして(kochirakoso hajimemashite) means "Nice to meet you,too"
These sentences are casual speaking.