If you're experiencing diarrhoea or a really upset stomach, try the
BRAT diet. It's a low-fibre diet that will keep you from pooping much, if at all, for a few days. If you're not suffering from diarrhoea then don't just do the above, as you'll be seriously constipated in a few days.
If you're feeling week and tired, try
Oral Rehydration Therapy to return the much-needed electrolytes (salt, sugar, and potassium) your body needs when it loses them to vomiting or excessive shitting. There's a simple recipe under the "Availability" heading on that wiki page I linked. It's cheaper than buying gatorade, and isn't made with HFCS.
As for how long it'll take to go away, probably 72 hours to 5 days. I suffered a bad bout of viral gastroenteritis about 2 years ago and it was nearly a week before I felt back to 100% normal. I only vomited the first night (woo, projectile vomiting in the toilet, who doesn't love splashback on your face?) but was seriously unwell for the next two days, and was pretty weak for the following 4 or 5.
If you still feel weak after getting some electrolytes into you tomorrow then go see a doctor and get some good rest. After 48 hours of simple food poisoning (ie. not a bacterial infection or something more long-lasting like viral gastroenteritis) you should be able to go to school or work. If your symptoms persist past 48 hours or get worst, then definitely seek medical attention.
Don't over do it on just water. Too much water on a very limited diet will slow your recovery process down. You need sugar, salt, and potassium to keep your blood chemistry working well to recover.
Here's hoping you feel well soon. Food poisoning sucks horribly, but at least /bg/ can keep you entertained.
PS: How was the Must concert? I missed the show in Vancouver earlier this week.