You'll probably feel a lot of pain as you start training, and most of it is ok, normal, and shouldn't worry you. Something you might notice if you stick for a while is that your skin gets used to jiu jitsu. It's a good thing.
Don't think too much about this for a while, but remember that you shouldn't crank the back of the neck, or what they call the cervical. Your teacher will explain this better.
If you're doing it to lose weight, I recommend you don't worry too much about what you eat on the first week. Only remember that you should avoid eating 30 minutes before the class. If possible, eat an hour before it, and don't over eat.
Jiu jitsu schools tend to be way more laid back and relaxed than karate schools and other traditional martial arts places, so don't worry too much about etiquette and rituals and not knowing what to do. Just be humble and polite, and when in doubt, ask.