How to interact with the audience?

I think you need to be more specific in your question focusing on your specific problem area. Your question is just too general.

To give you a general answer, just open your mouth and talk as how you would talk to your friends.

How to interact with audience when performing?

Its still too broad a question. Perhaps you could be more specific? Do you mean like in mentalism tricks? What kind of audience?

If asked how it depends on your trick I guess, depends on the mood of how you want the trick to be.