One thing that most of the writers ask is: how to create a good dialogue? While the structure of a story can be divided and formulated in many ways until it turns into a "connect the dots", a dialogue is harder to anatomize and find pratical solutions to write them well. But there is something essential to understand how to write good dialogues: you have to know what is it for.
When Lajos Egri wrote "The art of dramatic writing", the chapter of dialogues was written by one of his students (Jeanne Michael), and she has one of my favorite quotes about dialogues: "dialogues are the main devices whereby the premise is exposed, the characters revealed and the conflict prepared. The dialogue must be good, since it's the most apparent part of the story to the public". What I like the most in this quote is that it shows why the dialogue must be good (most apparent part of the story), at the same time it says how it can be considered good (expose the premise, reveal the characters and prepare the conflict).
Most of the questions about dialogues usually are related to "how to make the dialogue look natural/realist". The best way to make your dialogue look natural (or credible, realist) is to read what you wrote out loud. If you feel ridiculous reading what you just wrote, there's a big chance that your lines are artificial. But "natural" is a really subjective concept.
If there's one thing you can look to see if your dialogue is good enough is in the function he has on the story. If you seek to much for naturality, you take the risk of make a boring dialogue, just like a conversation where someone says "hi, how are you?" and the other says "I'm great, how about you?". There's nothing wrong on this. The only problem is that if you dialogue stays only on this and doesn't get anywhere else. Stories are about someone trying to get something. They are about a protagonist leaving from point A to get to point B. And the dialogues need to do the same thing.
When Syd Field wrote about the functions of the plot, he made a list with 8 things:
1) it sets the story in motion, 2) communicates the facts and infos to the public, 3) reveal the character, 4) establish the character's relations, 5) gives realism, naturality and spontaneity to the character, 6) reveal conflicts of the story and characters, 7) reveal the emotional states of the characters, and 8) comment the action.
We can basically summarize this from the first to the third item. They will reveal characters. And the second item is what we call "exposition". The characters will talk between themselves to expose infos about the story through the dialogues. But the most important one is the first item: dialogue will set the story in motion. If stories are a journey from point A to point B, everything in the story will move the plot ahead, and dialogues are included on this. This is the problem with the "hi, how are you?" "I'm great, how about you?" kind of dialogue: unless these dialogues reveal the characters or sets the story in motion, they are useless. About this, specifically, John Truby says: "many writers, in an effort to sound realistic, start the scene and progress slowly to the conflict. This doesn't turn the scene realistic, it turns into a boring scene.". To look natural is not a problem, to bore your public is. This is why there is many dialogues out there that are unrealistic but, at the same time, successful. Like House, Gossip Girls and everything that Aaron Sorkin wrote. Because they're exciting. People talking are like cars racing really really fast and every time that one of them answers is like when a car makes a perfect turn in high speed. Or give back an impossible ball in a tennis match. Because good lines are the things you wanted to say, but you couldn't.
You know when you have a discussion with someone, and two hours later you realize you could have said something different? When you think "Oh, I should have said (this) instead of (that)?". When your brain had time to process what happened and imagined the perfect answer to destroy the person in which you were discussing with. This is the kind of line that composes a good dialogue.
In the article of unspoken rules of good dialogues in the ScriptLab, the first one is: "there's no such thing as realistic or naturalistic dialogue". When we define what is a good line and a good time: a good line is the one who causes more impact, while a good time will always depend on the context, but the faster you make it, better. Because in real life we always speak measuring words. So we will always have an astronomic quantity of "uuhmmm...", "ãããhhmmmm..." and silences. A dialogue don't have any time for this because your public don't have any time for this.
When two characters talks in a story, it's because someone wants something. So make this dialogue to work in function of this. The function of a dialogue is to set motion in the plot, and the mold of the dialogue will reveal the character. So if the character is trying to seduce someone, everything this character says must be in function of this. But if a king or a farmer tries to seduce someone, the mold, the way, the vocabulary, the tactics they use in their lines will be different.
To sound natural is when you go to the bakery and speak to the baker. Your dialogue must be intelligent, efficient and sharp. If you have doubt about the quality of the dialogue, remember it must do at least two things, and to sound natural is not one of them: set the story in motion and reveal the character. If it does these two, you can start worrying about anything else besides them. But while your dialogue doesn't do these two things, it leave to be desired. So you have to throw it away and start it again.

Now get out of here and go start writing dialogues...