Listening in on people’s conversations is not acceptable. Except if you are a writer and studying the way people express themselves.
Next time you are in a coffee shop, listen and observe: how do people interact? How do people of the same age chat with each other?
How do adults speak to children?
How do children talk to each other; to adults?
Is anyone having an argument?
Try to jot down some interesting lines or comments.
Now create a fictional dialogue between two people you spotted in the coffee shop. Perhaps one orders, and one works there.
Or have to friends talk to each other at a table.
This will help you to create realistic dialogue.