Well, literally, you can translate "I am happy for you" into "我为你感到高兴“. But in oral English, this does not sound native. I think “看到你高兴我真开心” will be better. But generally, I will just say "那就行“ or "不错“ or "真好“ in this circumstance. I do not know how to explain but this is my first reaction. Hope this can help.

"你好吗“ sounds very "textbook"... Actually I think on many occasions "How are you?" should be regarded as（你）最近好吗? or（你）最近过得怎么样? rather than "你好吗?" which we rarely use... And usually we don't use "你好“ to greet our friends or people we have known well. We use 嘿 or 嗨 instead. 嘿 sounds like "hey" and 嗨 sounds like "hi". "我很好" is fine. But most of the time I say 还可以，还不错，不错，还行吧，挺好（的），挺不错的。When I feel extremly happy, I would say 棒极了！ （Couldn't be better!") ...... "I'm happy for you." = 我为你感到开心/高兴 or 我为你高兴.

我为你感到高兴 or 我真为你高兴

Okay, that is a simplified dialog. But I never know how to express that I am glad somebody is doing well. So can you help me with my Chinese translation? 祝贺你 or 恭喜你=congratulations. 100% native 我为你骄傲 if he is your close friend or your children. A：你好吗？-- We rarely ask this question. Just say 你好。To my close friends and collegues, I even never say 你好 B：我很好！谢谢。 A：哦，我为你高兴了。 no 了

Normally, we Chinese have a different way of expressing this meaning, mostly a subtler way. We don't have the exactly matching expression as "i'm happy for you", as i can remember, mostly in any occasions. People usually say, "o that's good, (嗯，真不错！）” instead to show them that they feel happy for the other person. This reminds me of another long-lasting question in my head which i cannot figure out a better translation than the literal one which seems quite not native. That is "i'm sorry to hear that" in English when you heard some bad news from another person and want to show your sympathy. Because we Chinese simply don't say this sentence in our daily dialogue. Normally we just say something good or irrelevant to try to cheer up the person.