First, good luck with teaching at a technical high school! I've heard that they're a nightmare and the kids usually have zero interest in English. Most of the kids who attend technical high schools are not aiming for universities and if they are, they're aiming for low academic-rated universities.
One of the new Korean-English teachers at my school this year taught at a technical high school for the first years of her teaching career. She said she loved teaching there even though the kids were challenging. We've often chatted about her experiences at a technical school compared to my current school (a specialised language school). In 9 years of teaching, this is the first year that she's taught girls and she's often commented that teaching boys is much easier.
Do you know if your school is co-ed or single sex? I'd guess it'll probably be single sex - and most likely boys - so it makes it easier to tailor all of your content. Try to think of topics that they can relate to. Think of things that you can do in every lesson to catch their attention - like starting with a riddle, a song, a short video clip, etc. Rather aim to pitch your content at a much lower level than what they really are - think very basic to almost no English. Work on the assumption that a lot of your kids will most likely only be able to say, "Hello" - expect the worst but hope for the best - and maybe, "How are you? Today is _______." If you pitch your lessons at a very low level and find that they're capable of much more than you expected, you can always make the tasks a little more challenging in the lesson provided you've chosen a topic that gives you some flexibility.
Try not to get too frustrated if they don't really respond in the first couple of weeks. Keep telling yourself that, at most technical high schools, English is not a top priority for the majority of students but that doesn't mean they won't like your lessons. Aim to make things fun - with one or two actual grammar concepts hidden in the content - than focusing on trying to teach them something. I'm not saying that this is what your lessons will definitely be like but, from what I've heard from a lot of people who teach at technical schools, there seems to be a pretty common link among students.
The first week of planning is going to be a shot in the dark - no matter how experienced you are - so choose things that you're comfortable with and that you think teenagers (probably boys) will relate to.
Good luck!
