oh my! Tomorrow I am going to be a teacher! (Substitute) Oh My!

That aside, concentrate on their learning what's been set out.

Here it's hard enough to get full-time teachers so requiring subs to be ceritified would guarantee no subs. In TX at least you must have at least 60 hours of college to sub. A degree gets you paid more, teacher certification gets you paid the most.

To go along with this, do not get flustered, appear flustered, or show any anger. If they try anything, do not overreact.
(Though my experiences with supply teaching was mostly at the high school level.)

...have fun with the little tykes

Do not make this a goal, at least on your initial placements.

Have you no experience at all in a classroom? Here in Ontario, supply teachers have all had gone to teacher's college, with a minimum of 9 weeks in the classroom.

What grade is it?

And Andy's use of the word "qualified" I would assume analogous with our "certified". To be certified as a teacher only requires a bachelor's degree with a handful of pedagogy classes thrown in, a minimal amount of in class training and a couple of tests. One could very easily be legally certified but hardly more qualified than any first-time sub.

Only in Scotland though - in England & Wales you can be completely unqualified and still do supply (though you do need a CRB check).

Good luck, though. The most important thing anyone has said here is to set boundaries early and firmly.

aim, one of the best pieces of advice I had when I was subbing is don't call for/send kids to the principal. A principal will always invite you back if you don't bother him/her with discpline issues. obv if there is imminent threat involved, do what's neccesary. Otherwise turn to nearby teachers for help. (definitely introduce yourself to neighboring teachers early in the day, they can be a huge help.)

Expect that the full-time and regular faculty will be somewhat distant, though some of them may be friendly. Discipline issues should be dealt with quickly and obviously. If you try to be exceptionally fair-minded, if you try to allow each little incident a 'trial' you'll get so bogged down in disciplinary minutae that you'll go nuts and the kids will get out of control.

There'll be a lot of 'this isn't how we do this...' stuff at first, and you need to get past that by seizing the reins of the class firmly.

The first day I ever subbed was a disaster. I'd taught elementary writing classes in a rural town prior to going into the substitution game. Decently well. But the school I got called into was an urban wasteland of a thing; and the kids were (prior to my arrival) out of control. I spent the entire day writing names on the board, yelling, trying to get kids to take their seats. I couldn't, honestly, get them to sit down! One of the little boys (this was a 3rd grade) actually ran away from the room on two separate occasions. The kids had under my attention a fist-fight and a few crying incidents, and I chalked every single thing to my own inability to control the room. It was partly true, and I still feel sooooo responsible. My errors were manifold.

Primarily, I'd tried to be 'nice', and understanding. Every time there was a complaint, an incident, or an act-out I'd freeze the rest of the class to deal with it. I'd give the culprit or complainer the exact attention s/he'd asked for, and the undivided attention that I didn't give the kids with good behavior. My goal was being 'fair' and not being effective, and that was fatal.

Additionally, the room I was subbing in was right next to the office. The kids were wildly loud, and so the school secretary would constantly yell through the wall at them. A goal of mine became quieting down the noise 'ssh, everybody lower your voice' rather than addressing the root cause of the yabber with something like: 'stop it, Iago' or 'Alyssa, take that pen out of your partner's ear.' I didn't realize how helpful specificity can be to little guys, and how worthless telling 'everybody' to do something can be.

I also had (first day subbing) an epileptic in my room. Nobody thought to tell me. Not even him. Further, he was the total class clown. So when, partway through the day, he flipped onto the ground I didn't know better than to yell 'as third graders we can stay in our chairs and not roll around.' I didn't know I was being totally offensive. And when I found out, I publicly apologized to him and spent the rest of the day trying to make it up. Not good.

Finally, I got cold-frozen in the staff lounge. I was a mid-year replacement building sub, and I was replacing a really popular woman on maternity leave. See, there are a lot of great, committed, and gifted teachers out there. I work with many, now. But there are also some adult-children who settled back into a juvenile educational system so they could finally boss around and control a classroom. Because their level of maturation topped out sometime around grade X. I took a long time to learn that they weren't singling me out to be cold to: they were just weird and not very nice.

high school science: one girl has a pet rat in her backpack, three boys climb out of the second-story classroom window and disappear.

middle-school computer science: all of the little balls have been stolen out of the mice so we can't actually use the computers. I spend most periods holding the door closed so kids outside roaming the halls can't come in to disturb us. Students shoot dice in the corner of the room.

elementary school: Only did one day in elementary ever so forgot to do things like pick them up from gym, etc. When I announced "You guys can go to lunch now." One girl, with tears in her eyes, said "Aren't you going to take us?" oh, sorry. bad: two boys who pretended to hump me from behind when I was bending over a desk. good: little girl drawing me a picture and asking if I would be their teacher forever.

While I was a full-time teacher, a sub next door came and grabbed me b/c she caught a couple of students selling pot. I had to march them and their fat baggie down to the police office and one ran off on the way. meh.

haha, you will be fine though Aim. Most of the year I spent subbing was pretty low-key and enjoyable. You get to experience different classes, different kids and, much like babysitting, you get to give the kids back when you're done! phew.

we are stocking up
on wine and ibuprofen,
heroin and cake

basically, act cool, don't yell unless you have to. i agree that sending kids to the office is something to avoid .i did it once, when a kid repeatedly threatened to kill himself-- i figured i had to take that seriously, so did it as a precaution rather than punishment. try to get the kids to explain what they've been doing to you (this is reviewing, nothing wrong with that!), and have fun with it.

don't worry about being too young, i was about 20 when i subbed and some kids i was teaching were 18 and 19. those classes i did enjoy subtly knocking the confidence of the bratty popular kids and bigging up the unpopular indie/goth kids.