Hi Libraryjoy,
I spent years on the sessional teaching treadmill, which means CONSTANT last minute class assignments. Honestly, I've had times I've been hired 2 days AFTER classes started, other times I've taken over a class mid-semester after a colleague went on sick leave, etc etc.
You're doing great! You have lots of time. This is your first time through, and so it is normal to panic about all the unknowns until you actually have a few weeks of experience under your belt.
Some things that have gotten me through:
**remember that especially the first week or so, the students are MORE nervous than you are!! You can breathe, act confident, and remember that you know more than they do about the course, because you planned it and you're teaching it.
**if the previous material that you've been given is out of date or overwhelming, just throw it out. You have no obligation to teach as the last person did. Follow the advice given here to pick a textbook, get teaching materials from the publisher. It is way easier to simply read the textbook, learn the info for yourself, and then add the context that you're aware of from your extra years of experience/knowledge (I don't mean this in specialized way, just life experience).
**Some people get anxious about projecting authority in the classroom, and so try to look like experts and stress when they are not. I get this, especially if you are only a few years older than your students. But I am a huge believer in being confident, but also transparent. So I act professionally, say a little bit about my credentials and what's brought me to teaching, and then I say with much enthusiasm that I will be teaching this course for the first time, and I look forward to learning WITH them about the course, look forward to hearing their feedback, etc. Sometimes this is reflected in one or two evaluations badly, but more often they are also patient, and they also treat the course as a co-learning experience. I think the key is lots of enthusiasm and obvious devotion to trying to make the course go as well as it can. But it opens up flexibility when the inevitable thing I hadn't anticipated comes up, and I can say, "D'oh! Thanks so much for that question/observation/pointing out that error, etc. I'll get back to you, and definitely remember that for next time!"
Lastly, I would just say that I have often gotten myself into a real tizzy about "OMG I don't know anything!! I shouldn't be doing this!" before the course starts. I teach English, so it might be an ambitious theoretical question, or a novel I haven't taught before etc. Then, once I get in the classroom, I remember that, oh yeah, that doesn't matter. All that matters is that I know more than the students about this topic. Which I do. Inevitably what we actually need to do in the classroom is pretty basic, as others have said: Go over the readings, make sure they have understood the concepts, give them some time to talk to each other about the implications of those ideas/concepts, and then give them an exercise to make them apply the concepts. You can do that.
I remember supporting a young colleague who took over a course not in her area a couple of years ago. She would ask me, "what are the most important points that I should teach about this novel?! I've been trying to figure that out, but I don't know this area well!" My response inevitably was, "Ask the students!!" Remember that you don't have to provide all the answers. You are just the facilitator and guide. Help them identify the questions, and make them do the work to figure out the answers. :)
Hope that helps! You can do this! You don't have to be perfect! You will find your groove after the first couple of weeks. Keep us posted along the way!