这得看对方是什么身份，什么职业。

if they are strangers :D for elder people, we often use "先生""女士" for peers, we just call them "同学"

if u meet a girl， u can say：美女（mei nv）mean beautiul girl if u meet a guy，u can say：帅哥（shuai ge）mean cute guy

Mr., = 先生 Mrs. = 女士 all others are by position, e g 经理， 老师，校长，等等

Most time, when you meet an elder, "您好" is unavoidable. Then you can choose "先生", "女士" depending on the gender. We usually ask a stranger's last name with sentences like this "请问先生您贵姓" to shorter the distance. If we know his/her last name, then we can use "张先生",“王女士” instead of "先生" to show the respect. Normally, "你好" is ok, and "hey, 哥们（兄弟）" "帅哥" “美女” are commonly used among young people. If you cant judge how to choose a title when opening a conversation, you can just say "你好（您好），。。。" and begin your topic.