SETTING THE MOOD
Notice I didn't say setting the scene. This isn't about providing the reader with a layout of the room. It's about using the crusty tablecloth, peeling wallpaper, and the floor --not swept in months-- to show the reader that this place is unkempt or abandoned.
You shouldn't tell the reader anything, as they are exploring this world you've created, and letting them discover it through your storytelling is what reading is all about. They will probably expect that a cabin in the middle of nowhere is ramshackle. But when the contracted lawn-care specialist (who just so happens to have been kicked out of his girlfriends apartment, has little cash on hand and is desperate for a place to crash) realizes during his weekly mowing that a certain house in an upper-middle-class neighborhood has a back door that's been standing open for his last three visits...and he justifies to himself that poking around might be okay...and he discovers the house inside is nothing like he anticipated...
Now you're building a mood, a mystery, and a story.
Your style and tone have a lot to do with creating that mood. A murder mystery can be sinister like "What Lies Beneath" or it can be light like the average episode of "Murder, She Wrote." The choice is yours, it will be dictated by the characters you choose to create and populate your fictional world with, and they will by their very nature (which you give them) create the tone and mood of your story.
By what Point of View you choose, what you allow them to see, and how they react. You will build your character, as well as mood, by careful attention to your language.
You may be prone to writing humor. Or you may be romantic. You may have a knack for the dark arts. But, for the purpose of learning to stretch your writing muscles, try this exercise with a few different scenes to get a feel for MOOD:
Pick a scene from either your WIP or at random from one of the books near you. Read it, identify the mood. Now, rewrite the scene, making the mood the opposite of what it is now. I'm not saying to change it in your WIP, only to study what you did (or what the author did if you use a random scene from a book) and actively make decisions about how you would alter the scene to make it read differently in MOOD only. Let the dialogue remain as it is, if there is any.
Good luck! Let me know what you thought of the exercise!