You are not expected to answer this with any specifics, it is a ritual greeting.
"Very well, thank you!" is a formal, polite reply. You can append "And you?" to show interest in the other person, and continue the conversation.
"Fine, thanks" is a very common reply (just like 元気です). You'd say this to anyone when you don't actually want to continue the conversation, like to a waiter in a restaurant.
You can vary the response depending on who you're talking to, and how thrilled you are to be talking to them.
"Great! How are you doing?" is a very enthusiastic response, and asking the person how they are shows that you're eager to continue the conversation.
Normally you're supposed to answer in a positive way, but that too can be varied depending on your relationship.
"I'm hanging in there." -- you might say this to a coworker; it indicates that you're having a rough day.
"I've been better." is what you can say to a friend or acquaintance when you want to tell them why you're not having a good day; it indirectly invites a question from them asking you what's wrong.