For yoga nidra, you want to be as relaxed and comfy as possible. Think lying down in a nest of pillows and blankets - ultimate comfort. Do your best to limit outside distractions, although once you get used to "dropping in," you won't even notice outside noises after a while. 



I hope your teachers find it relaxing! Let them know they don't need to worry about being perfectly still throughout the practice - it's better to let yourself adjust if you're uncomfortable than to fight it the entire time and miss out on relaxing. After you've practiced a few times, you'll know what you need to do to be comfortable enough to remain still the whole time (I still fidget occasionally!)