In general, what should you expect to do as substitute teacher?
First, arrive early. “Early” is time enough to find and register at the office, to obtain a key to and find your classroom and to find and review the classroom teacher's plans for the day. Often, you will also have to identify the location in the classroom of materials for the day: audio-visual equipment, supplies, and even the pages to be covered in textbooks. Be aware, however, if you arrive too early to prepare, you may not find offices, or even exterior doors, open. Also, don't be surprised if neither you, nor the office personnel who shows you to your classroom, can't make the key to your assigned classroom turn in the lock. Many classroom locks are temperamental, and their keys have to be jiggled or their knobs have to be turned just right to allow entrance into the classroom. When entrance fails, usually a janitor can open the door. Still, such idiosyncrasies take your time and can cause you to be unprepared when the bell rings to start instruction--so come early.
With luck, and we'll assume you have it, the classroom teacher will have, in his mailbox or on his desk, prepared lesson plans complete with class lists, seating charts and the lessons for the day. Even if the regular teacher was not expecting to be absent, an emergency folder of general lessons should be somewhere on or in the his desk. Review and try to understand classroom rules, procedures and lessons. Look around the room for emergency fire and other evacuation signs if instructions are not included in the packet. (Especially, in spring and fall, trial emergency plans can be tested at any time.)
If you can't find class lists, call or go to the main office for them. They will be on file. If there are, heaven forbid, no lesson plans and you haven't brought along emergency materials, go to the media center (library) for the day's newspaper. Find articles that pertain to the subject matter of the day and present those as the lesson.
Nevertheless, classroom teachers usually do leave lesson plans, rules and procedures. Follow them! Chances are that the regular teacher has spent more time preparing the lesson for you than for herself on a normal day; don't insult her by going your own way. Also, follow her rules and procedures if, for no other reason, than to provide a sense of stability to students. Even if you are critical, do not criticize or mock the lesson plans, the rules and procedures, or the classroom teacher. Do as you are told. Also, do as the classroom teacher is told; if she is scheduled for hall duty, you are scheduled for hall duty.