Dialogue's fun to write, but it can also be really tricky for new writers.
For me, I see dialogue in writing as an idealization of speech. If you think about it, written dialogue is perfect in a sense. People in writing don't correct themselves, they don't repeat themselves, they don't make mistakes in pronunciation, they don't stutter or say things like "like," "um," and "uh."
True, we do sometimes use these elements in our dialogue, but it's always done with a purpose. Written dialogue doesn't have to and shouldn't emulate all aspects of speech.
Here's how I typically tackle writing dialogue:
Whenever I need to write dialogue for a scene, the first thing I'll come up with is a specific thing that I want a character to say. This is usually an important plot point, an emotional statement, or (even more frequently) the punchline of a joke. 
From here, I'll retroactively create the conversation so that what the characters say eventually leads to someone saying the line I want them to. Occasionally I'll type out fragments of conversation, which I'll then try to "connect" to each other by transitioning subjects.
This isn't the only way I do it, of course, and it's not the only way you have to. Sometimes I just let conversations flow on their own and see how they turn out. That's the biggest thing for me - flow. I like dialogue that is fluid and clever.
There's got to be a logical flow to what the characters are saying. I want to believe that when a person replies to someone in a conversation, they're saying the only thing they could have said in that moment. It should feel like its coming from a character, not from the author.
Granted, though, it's almost impossible to avoid implementing artifice in dialogue.
What I mean is this: there is NO POSSIBLE WAY for you to create dialogue that will seem authentic to everyone who reads it. There will always be someone who thinks and interprets differently than you. This is the same reason why there is no such thing as universally good prose.
Don't believe me? Go look up one star reviews for Hamlet. 
So there's no real way to ensure that your dialogue is objectively good. But there are ways you can make it BETTER from a technical perspective.
Here are my biggest tips:
1) Adjust the speed of the dialogue to suit the needs of the story!
Speed is primarily controlled by your use of beats. Fewer beats means faster dialogue.
Another way to affect the speed is by varying the length of sentences. If you want your characters to have an intense back and forth argument, for instance, having them speak short sentences one after another is a good way to do this.
2) Use your editing phase to make different characters sound different from each other!
You're almost never going to get ideal dialogue on the first shot. If you want your characters to sound different from each other, the best way to do this is to edit the dialogue in post. Go through and check to see if characters are using similar phrases, words, sentence structure, etc. and make changes accordingly.
3) Think about WHY your characters are saying the things they're saying, not just surface level!
Remember, your characters should have history and depth! You're the magician on stage. Your job is to convince readers that your characters have more complexity behind them than they may have in actuality.
Consider whether or not your characters have certain pet phrases. Why do they say these things? Are there forms of speech your characters are trying to emulate? What are your characters thinking about as they talk? What emotions or goals are driving the dialogue?
There's so much more to be said, but I'm getting a little tired of writing this post. I'll leave you all to consider this advice and judge its merits. 
Dialogue's fun to write, but it can also be really tricky for new writers.
For me, I see dialogue in writing as an idealization of speech. If you think about it, written dialogue is perfect in a sense. People in writing don't correct themselves, they don't repeat themselves, they don't make mistakes in pronunciation, they don't stutter or say things like "like," "um," and "uh."
True, we do sometimes use these elements in our dialogue, but it's always done with a purpose. Written dialogue doesn't have to and shouldn't emulate all aspects of speech.
Here's how I typically tackle writing dialogue:
Whenever I need to write dialogue for a scene, the first thing I'll come up with is a specific thing that I want a character to say. This is usually an important plot point, an emotional statement, or (even more frequently) the punchline of a joke.
From here, I'll retroactively create the conversation so that what the characters say eventually leads to someone saying the line I want them to. Occasionally I'll type out fragments of conversation, which I'll then try to "connect" to each other by transitioning subjects.
This isn't the only way I do it, of course, and it's not the only way you have to. Sometimes I just let conversations flow on their own and see how they turn out. That's the biggest thing for me - flow. I like dialogue that is fluid and clever.
There's got to be a logical flow to what the characters are saying. I want to believe that when a person replies to someone in a conversation, they're saying the only thing they could have said in that moment. It should feel like its coming from a character, not from the author.
Granted, though, it's almost impossible to avoid implementing artifice in dialogue.
What I mean is this: there is NO POSSIBLE WAY for you to create dialogue that will seem authentic to everyone who reads it. There will always be someone who thinks and interprets differently than you. This is the same reason why there is no such thing as universally good prose.
Don't believe me? Go look up one star reviews for Hamlet.
So there's no real way to ensure that your dialogue is objectively good. But there are ways you can make it BETTER from a technical perspective.
Here are my biggest tips:
1) Adjust the speed of the dialogue to suit the needs of the story!
Speed is primarily controlled by your use of beats. Fewer beats means faster dialogue.
Another way to affect the speed is by varying the length of sentences. If you want your characters to have an intense back and forth argument, for instance, having them speak short sentences one after another is a good way to do this.
2) Use your editing phase to make different characters sound different from each other!
You're almost never going to get ideal dialogue on the first shot. If you want your characters to sound different from each other, the best way to do this is to edit the dialogue in post. Go through and check to see if characters are using similar phrases, words, sentence structure, etc. and make changes accordingly.
3) Think about WHY your characters are saying the things they're saying, not just surface level!
Remember, your characters should have history and depth! You're the magician on stage. Your job is to convince readers that your characters have more complexity behind them than they may have in actuality.
Consider whether or not your characters have certain pet phrases. Why do they say these things? Are there forms of speech your characters are trying to emulate? What are your characters thinking about as they talk? What emotions or goals are driving the dialogue?
There's so much more to be said, but I'm getting a little tired of writing this post. I'll leave you all to consider this advice and judge its merits.
