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.