Natural-sounding dialogue

Some tips for natural-sounding dialogue:

1. Imagine you are your character, at that point in the conversation. What is running through your mind? How did the other person make you feel?
2. Try to imagine the speaking character's "voice" - what do they usually sound like? (Arrogant, shy, educated...) Imagine you are speaking with their voice.
3. What would you say if you were that character in that situation? That's your dialogue.
4. Visualise the conversation. What is the speaking character doing? Leaning forward because they're interested? Looking to the side because they don't really want to be there? Dangling their keys because they're nervous or bored? Focus on the body language which catches your inner eye and seems to say something important about the person, and use it as your beats.

If this seems hard or impossible to you, maybe it helps to pay attention to the speech patterns and body language of ordinary people in everyday situations, until it becomes intuitive.