If someone asks you お元気ですか, simply respond affirmatively with 元気です and perhaps return with a similar question: "Aさんは？”
Make sure that you never use Japanese equivalents for "you," as that can come off as rude. For example, you would most likely not say あなたは？ Instead, you would use there name plus さん. For example:
If there name is 田中: 田中さんは？
Where the conversation goes from there is dependent on your Japanese level and your mutual interests.