I'm not at a bushiban any longer, but I was a while back. Arrived almost 3 years ago. Got in on a Friday, work permit was finished up, started on the Monday afterward. It was good. Small classes, mostly one-on-ones, and the kids were nice. I didn't work for any of the bushiban chains, just at a friend's school, so I didn't have to deal with any adversarial relationships with my boss.
I wasn't really nervous. I had to prepare my lessons from scratch, but I was okay with that because I don't like teaching from a textbook. I guess you could say I was thrown in there, but if you know your subject and how to deal with kids you'll be fine. Wasn't that hard either; I just prepared my lessons, chatted with the kids, and they liked me, so it was good.
Remember that the kids probably don't want to be there, even though they will be shy and polite to you, because they study at least two hours after school every day, in addition to hours of homework. Anything you can do to make it enjoyable for them will be appreciated. Ask them about their days, be interested in who they are, don't let them feel like they're in prison. Talk to their parents if possible, there are some cool parents out there and having a parent on your side will make things eight million times easier.