One of the biggest complaints I've heard from new writers over the years is that writing dialog is difficult. Some people have a natural "ear" for it. They are the kids who when playing make believe would tell you what to say..."Now you say, no, don't go!" And I'll say..."I have to go to save the farm." Then Suzy says, "We'll all go together." You remember those kids--perhaps you are one. If so, this blog is not for you.
If, on the other hand, you are a writer who grits their teeth every time you must write dialog, here are a few tips to train you writer's "ear."
1) Sit in a public place with pen and paper or notebook computer in hand and eavesdrop. Write down what people say. Pay attention to how they say it. Notice the difference between two guys speaking, a guy and a girl speaking, and two girls. People use different words, tones and styles of speech depending on sex and age of their partner in the conversation.
2) Pull your favorite fiction novel off the shelf and flip to the scene that really worked for you. Write down only the dialog--no tags. Now read it. Can you tell if it is a male or female speaking? Compare ficiton dialog to the notes you took of real dialog. Yes, there is a difference. Fiction dialog must have meaning in each word. In your book, you don't waste time with hellos, goodbyes, small talk. You must zero in on the meat of the discussion--the reason to talk at all and ensure you get that in the dialog. Pay attention to how this author did it. Take the time to flip through a second or third book--copy a snippet of dialog. Compare. Some of the best textbook examples of writing are found on your favorite books shelf.
3) Write your scene. Then highlight the dialog of the characters with a different color for each. Read only the dialog aloud. Can you tell which character is speaking? Can you distinguish their sex and age? If yes, congratulations you are successful. If not, go back and try again.
Writing dialog takes practice, patience and a trained ear. Think of a successful singer, they have trained their ear for proper pitch and expression through years of practice. While you may not have a coach with you, you will find yourself in public places. Take a moment to pay attention to the dialog around you. After a while it will become second nature and your characters will speak to you. Good luck and happy writing!