Too much to write in one comment, but in general, the best thing you can do is care about the topic and your students. They can tell if you care, and I find that being passionate helps motivate people to learn stuff they might otherwise not enjoy. Anything in particular you want advice/insight on? I'm happy to answer questions!
Thanks! What was it like to first enter the classroom? What was your first impression of your students? Did it change as you got to know them? What was it like to finish the first day of teaching?
First time was pretty nervewracking, and I still get anxious before teaching, although once I get into the swing of things I calm down. My first day I learned an important lesson: performance anxiety is real. I was doing some basic probability analysis that I figured I could do off the top of my head and made an absolute mess of it and had to start over! So now I make sure that regardless of how much time I have to prep I make notes that feature fully-worked examples and exactly-worded definitions. (Much of the time my notes are just that and I can do the motivation and exposition bits off the cuff.) I was only a couple years older than most of my students when I started and I was concerned that they'd doubt my knowledge and argue about stupid stuff, but that turned out to be entirely wrong. I'd given plenty of presentations in classes before, so I was at least a little used to standing in front of a classroom, but teaching felt different because the students were really paying attention to what I was saying for a change. Definitely a sobering moment when you realize you could say just about anything and a room of 50 people would just take your word for it! A lot of my students have been very smart, and I really love the oddball questions they come up with, even if I don't always know how to answer them. Teaching takes a lot out of you--even though I don't feel tired while I'm in the classroom, I pretty much have to schedule a half hour or so after class to sit and drink coffee and do mindless tasks because I need a bit of time to mentally recover.
What is it that you get nervous about before classes? When I think of failing my students, the reason is always being unable to convey the knowledge that I have; that I'd get flustered and wouldn't know how to say it so they'd understand. I know I'm good at using my knowledge. I feel like I can't be sure unless I give it shot whether I'm good at sharing it. And, if you don't mind, I'll reply to your second comment here, as well. I'd like to get into teaching in a uni or a similar higher-education facility because I find it easier to work with more mature people, mentally. I'm not sure if my degree would allow for it, so I aim at high-school level. Any advice on dealing with teenagers? The caveats, the ways to interest them?.. I know I'm getting that from the future courses, but I'd also like some experience from the field itself.
I think it's just general public speaking anxiety but I haven't thought too much about it since I usually am busy prepping right before and so long as I don't just sit there and think about my feelings I can keep calm. Sharing knowledge is absolutely a learned skill. If you can, get involved with tutoring as soon as possible, even if it's just informal hanging-out-in-a-department-lounge-and-talking-to-other-students tutoring. You get one-on-one time with students and most of them are comfortable saying that they have no idea what you mean by what you just said. It'll help you develop the ability to explain the same idea a bunch of different ways. I'm a grad student so I just teach college students, which has a couple of advantages: they're (slightly) more mature than teenagers, and they're in school to some extent because they want to be there. So, I can't speak to dealing with teenagers in particular. I mostly teach math and programming. I like to motivate material by introducing a question students don't have the tool to answer yet or by drawing analogy to something they already know about (e.g. when talking about rational expressions I'll start by talking about how polynomials kind of-sort of behave like integers, point out that we can make fractions from integers, and then get in to how rational expressions work). Another thing that's fun to do is to talk about the history around an idea -- who made it, what were they trying to do, why is it the way that it is, etc. People like stories and having a little narrative to go along with an idea helps them remember it better and might even help them see the bigger picture. Just about anyone can read Wikipedia and get factual knowledge on a topic -- it's your ability to provide context and build relationships with other ideas that is really valuable to students. Jokes and terrible puns are also great!