It depends on the severity of your symptoms and on the person. I get stomach flu all the time and usually I can't eat or drink anything for the first day or so. On a good day, I can eat soup or jello, sometimes crackers.
Even if you can't actually eat anything, try to keep hydrated or you'll pass out from dehydration (I know this from personal experience). Gatorade or Pedialyte (yes the kids drink) are good because they have electrolytes. Water, plain tea, juice and ginger ale are okay as well.
Don't eat or drink anything with dairy, as that will upset your stomach even more.