You should respond also with "dzień dobry". You can also use for example "Witam" when you are a host or "cześć" when you meet a younger person or a friend.
We use "wzajemnie" or "nawzajem" in response to someone's wishes, on Christmas, birthday etc. It means "the same to you". For example:
- Życzę Ci powodzenia na egzaminie. - "I wish you luck on the exam"
- Wzajemnie. - this means that we also wish the first person good luck on the exam.
or:
- Wesołych świąt!
- Wzajemnie.