If you've had the stomach flu, you may think you have never, ever, ever been sicker than that. It's the worst feeling in the world. The biggest thing to do is attack it quickly. If you let the stomach flu or the stomach virus prolong too long, you're going to get dehydrated, and actually get much sicker.
You may either have nausea and vomiting, diarrhea, or both. In both cases, you really lose a lot of fluids, and you can get very sick from it. You really need to hydrate yourself with products such as Pedialyte, diluted juices, diluted Gatorade, or Vitamin Water. All of those have electrolytes and water. That really helps to hydrate your body.
If you need to hydrate yourself, and you don't have any juices, Gatorade, or Pedialyte at home, you can make your own electrolyte solution by taking some water, a pinch of salt, a little bit of sugar, and a little squeeze of lemon. That's going to help you rehydrate you, and give you a really good rehydration solution.
Straight water doesn't work that great, because it actually makes you more nauseous, and may make you vomit even more. If you can get the electrolytes and the water in there to try to rehydrate you, you're going to do well with it. Trying a little Pepto Bismol will help also with those symptoms of nausea, vomiting, and will definitely help with the diarrhea.
If it doesn't go away between 24 to 48 hours, you really need to seek medical attention, because you can get really sick if you get too dehydrated.