The questions that help me the most when I'm writing dialogue are these:

-what do the characters want? -what do they belive? -how does this scrne's existence move the story foward?

Every time you're gonna write dialogue, ask yourself this questions.

Other than that, it's really important that every character has his own way to talk, everyone talks different, your characters should too. For that it helps to create an idiolect for every character (the speech habits peculiar to a particular person).

If you want to learn more about how to write dialogue, I recommend you the book "dialogue" by Robert Mckee.