Dialogue is what the characters say aloud. It helps brings them to life.
It should sound realistic - read it aloud to see if it does.
How the character speaks is part of the way we get to know them. In a short story you have limited space, so use dialogue wisely.
Put inverted commas (" ") round the exact words the character says and start a new paragraph for each new speaker.
Try to have a balance of dialogue and description. Description should have a point in the story - it should not just be filling up space.
Descriptions of buildings or scenery can create atmosphere - for example, weather can be atmospheric. Descriptions of characters can reveal something about their personalities.