Hi there,
Well, it depends on the person you’re responding too.
If that person is a close friend then just respond casually with “Hi”.
If that person is an older one, be polite and respond with “good morning/day/evening”.
You can also go for the basic “Hello”. It’s about showing that you are open to discussing with that person so don’t lose yourself in the thought process of what to answer to someone saying Hi to you.
Be confident and respond with a friendly Hello or Good Morning etc and see where the conversation goes. Sometimes is just about saying Hi to someone and then going on with your day, it doesn’t always start a conversation.