if you really want to get scared read angry white pyjama's, this ones still giving me nightmare's but if you can read that and not blink you will be ready for anything a dojo can throw at you.
which to me honest at times will seem like the mat! your outlook on life will change from 90' to 0' in about a second, its the most fun you can have lying on your back (With the exception of one or two things)
and then you get to do it to your partner (gee this sounds bad)
btw if you find yourself cringing when sensei shows a move to the class dont worry its normal hands and arms actualy *DO* bend that way honest
i think whatever dojo you goto you'll have a blast