Write whatever comes to your mind as fast as you can. Get a feel for what the scene is about before going back to fine-tune.
Read what you've written out loud to see if it sounds like real people having a conversation. This is a dynamic way to learn how to write authentic dialogue.
Use silence to break up the monotony of constant chatter. When used correctly, silence can be more powerful than words.
Employ interruptions in the scene. People usually speak over one another, or they are distracted before they can finish their sentence.
Drop words now and then to mimic real speech. For example, a character might answer, "Hard to say," instead of "It's hard to say."
A person's background can come through their dialect. When learning how to write dialogue, it's important for the character's speech to reflect where they've lived. For instance, use southern jargon for someone living in the south.
Think of an unusual word or phrase that each character will use often. This certain word or phrase will only be used by that one character throughout the story.