First, you'll need to let the reader know that the scene is taking place inside. To do so, you'll write INT. (all caps followed by a period) at the beginning of your scene heading. If your scene were taking place outside, you'd write EXT.
Name the location of the scene next. If it's taking place in a character's bedroom, you'd write INT. KATIE'S BEDROOM.
Next, tell the reader if the scene is taking place during the day, or at night. If it were taking place during the day you'd write: INT. KATIE'S BEDROOM - DAY (leaving a space before and after the hyphen).
Two spaces below your scene heading you begin your scene description. Your scene description should be present, active, concise, and only describe the absolutely essential details of the room. For example, if an opened notebook on Katie's desk will become an important feature of the scene, you might write in your scene description: "An opened notebook lies on the desk." Also, if there is someone already in the room, make sure to note her presence in the scene description, and capitalize her name (if this is the first time she is noted in a scene description; her name does not have to be capitalized in subsequent mentions).