There are basically two meanings to the phrase "How's it going":

1. Hello.
2. How are you?

If someone who you know asks "How's it going?" while walking past you or working on something busily, it's similar to saying "Hi". You can respond with:

Hey, how's it going?

Good morning.

Fine, thanks!

If you're sitting down with someone and they ask "How's it going?", then they're trying to start a conversation. You should answer the question and add some other information or questions:

A: How's it going?

B: Not bad. I've been so busy this week, though. It's crazy. How are you doing?

"How's it going?" is an abbreviation for "How is it going?".

This phrase is a little casual, but it's OK in a lot of different situations. If you want to sound more formal, you can say "Hello" to greet a person, or "How are you?" to start a conversation.