Well first you need to realize what your story is focused on. Are you focusing on the mental ward itself or this specific patient? If the patient consider this, are you writing it from the patients warped perspective? Or are you writing it as if we were looking through their eyes. How long are you going to make it? If its a short story then focus on the characters interactions with the world around him. If its a bit longer build the character a bit. Don't even neglect building the character completely though, because then its difficult to sympathize with them.
Most importantly though, if you are just writing this for fun, then just write whatever you want to write. Just have fun and let your thoughts flow on to the page, screen, papyrus, stone tablet, ect.