Wow! You're working yourself up into a bit of a state, and rather needlessly too I'd say. Firstly, a good school would recognise that as a new teacher you'd need a bit of extra support. In all the schools I've worked in there's been a DoS/ADoS or both plus senior teachers who've been able to offer advice. In any case, inexperienced teachers are often asked at interview if there are any levels/age groups they are not comfortable teaching. As a complete newbie I'd ask to avoid Beginners and Proficiency, as well as Cambridge exam classes.
When it comes to knowing your subject, note that if teaching from prescribed coursebooks they're likely to come with an accompanying teacher's book. While some teacher's books are undoubtedly better/more useful than others, most will give you some idea of how the unit/section is intended to be taught. When it comes to the grammar, I'd advise studying the language point before the lesson. You can either refer to grammar books such as the ones used on the CELTA, the internet or you can do as I did in my first year of teaching and use a low-level course book to brush up on the grammar. Most course books have a grammar section at the back (intended for students obviously but I found them quite useful!).
I'd also consider applying to schools with a programme of in-service teacher training, such as those in the International House network. I cut my 'teaching teeth' there and, on the whole, I found the training sessions very useful. Note that not all IH schools are cut from the same cloth so tread with caution.
FWIW, my first academic year was pretty tough initially. This was because I was teaching age groups and levels I'd never taught before and I was expected to plan my own courses (using prescribed course books, but it was still very challenging for me). I'd say it took four to six weeks for me to stop feeling like a little girl play-acting and start feeling like a real teacher! That's not to say that there weren't problems after that - one of my classes was incredibly difficult - but on the whole I felt quite positive about teaching.
I'm now in my third academic year and I'm happier than ever with my decision to become a teacher. Not only am I confident that I now know what I'm doing (and why!), but between summer schools and academic years I've been lucky enough to teach in five different countries. And that makes the struggle of the early days worthwhile.
If you have any other questions, please don't hesitate to ask.