How do I add details to my story or know which details to add?
If by details you mean descriptions of characters or locations, or actions of a character, choose only those details that will tell us the most about a character, location, relationship, etc. It’s more important (and more interesting) that we know your character is a little paranoid than that he has brown hair or that we on a hot, windy beach and not inside a cramped closet. But remember to show us the details in what the character says and does, rather than just saying, “Joe was a little paranoid.”
If by details, you mean you feel your story is too short and you want to add stuff to make it longer, stop right there! One of the best pieces of advice I learned is that every word/sentence/paragraph/scene/chapter counts. Each of those elements has a purpose. If you feel your story doesn’t have enough heft, check out Finishing a Story or Book for some things to think about as well as some of the other links on the Writer’s Hangout page.
Author Elmore Leonard once said, “Try to leave out the parts that people skip.” In other words, write the good stuff, the stuff that grabs readers’ attention and holds it!