Hopefully there are good lesson plans. There should be a seating chart so make sure the students are in assigned seats. (They may try to buffalo you). Know the policy on cell phones in class. Take attendance every hour. Know the beginning and end times for each class period. Circulate the room. Depending on the class, I like to chat with the students about their lives, sports, work, etc. Know the policy for misbehavior and the office phone number. Be firm but approachable. I usually find it easier than elementary. You usually have one prep hour with no students. I usually bring a book to read or crossword puzzles. Hopefully this is helpful.
be upbeat and professional. your mission is to hopefully continue the flow of learning, so if there are plans, implement them as well as the situation allows. dont let technology issues affect too much, as its common for it not to work for subs. once the students know you are not just a babysitter, it will probably be fine. most teens are more regulated and capable than we give them credit for. nobody liked the crabby old mean substitute, and they dont need a friend, they have plenty, so find a good spot in the middle somewhere and help them get to where they are headed. I tell my students that I am there to serve them, they are important to myself and society, and their job is to build a foundation for their very important lives. they will often work up to that ideal. many of my high school students have been hearing me say that since grade school, but I seldom ever get a rolled eye or a snarky reply. they appreciate that an adult respects them and sees their worth.