I had a level 2 ultrasound at 18 weeks. I didn't have to prepare at all for it. Call the dr. office where you are having it done, some places like you to have your bladder a bit full, depending upon how far you are.
Mine was 45 minutes, they measured everything, showed the gender, weight of baby, size of baby and blood flow.
Mine took only 15 minutea. You dont need to drink water. They asked me if I needed to use restroom. Good luck.