Each place and each therapist will always be slightlly different. Sometimes on the paperwork they will ask if there's any spots that you don't want massaged (such as tummy/chest etc), other times they don't ask but you can always specify. See if your therapist does the decolletage while you lie on your back. It's to die for. They do this sweeping thing from your forehead, down past your jaw, behind your neck and then around your decolletage. I could seriously just lie for an hour with that alone.
The disposal undies are optional. I usually keep my own on and they will keep you covered with towels etc. When doing your back, they will pull your undies down just a little bit and tuck the towel in - that's normal. Feel free to tell them if the pressure is too light or too heavy for you and they should adjust it for you.
Don't forget to drink plenty of water afterwards for the rest of the day, as massage releases toxins from your muscles and you need to flush it out of your system. Otherwise you can end up feeling like you're coming down with the flu if you don't drink any water (made that mistake once).
Hopefully you get a great, nurturing therapist. Enjoy!