I had the amnio done 2-3 weeks ago. They told I don't need to do anything for the prep going in to the office, they didn't want me to have a full bladder, either. But they wanted me to be on 24 hour bedrest and take it easy for as far as physical activity for 48 hours. They also wanted me to make sure that I have a ride home and would not be driving myself, but honestly I think those are just being cautious, because I felt fine after the procedure was done. It only takes 10-15 min including the ultrasound before hand, prepping and soaping the area, getting the sample, taping you up, etc. We sat longer in the waiting room! They sent me home with a sheet of what symptoms warrants a call back to the dr's office and it was fever, vaginal fluid leak or any sort of spotting. The procedure is not painful at all, so don't be scared.
ETA: I forgot to answer your question about food, yes I did eat normall, mine was scheduled at 11:00, so I had breakfast and a little snack at 10, I had soup for lunch but because I just wasn't very hungry, but ate a normal dinner.