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!