These are common greeting phrases. In America, we usually just reply, "Fine", "Good", or "Ok" no matter how we really feel. In reality, a passing stranger or acquaintance doesn't want to know how you really are - it's just another way to say "hi". If a closer friend asks, however, you could reply the same, or if you aren't doing well, you could tell them about it. Example: John: "Hey, how's it going?" You: "Not so good. I've got a huge project due at work and I'm not making any progress." John: "Sorry to hear that. Do you want to get some lunch?" You: "No, I've got to keep working." John: "Ok man, see you later." If a REALLY close friend or family member asks, you could go into greater detail, depending on how much you want to share with them.

@Phil Thanks for the explanation! Now I can say Hi confidently:)

You're correct.. these phrases can be used just as simple greetings. BTW, the word "hi" was originally a reduced pronunciation of "how are you?"