Best to eat something now before you start fasting at 12 midnight. If not you might feel hungry or faint tomorrow as the test takes quite some time.
Sips of water is allowed even though you're fasting.
There will be 3 blood tests taken. After the first blood test, you'll be given an orange drink to consume within 10 minutes. Don't rush it down as you'll have to redo if you vomit. Second blood test will be one hour after the first and the third, another hour after.
During this whole time, you are not allowed to eat anything. You can only eat after your third blood test is done.
Results will not be out immediately. If nothing is wrong, you will not be notified. They will only call you, within a day or two, if your results shows GD.
I'm not sure about the procedures if confirmed to have GD since I don't have it. But your doctor will advise more about it.
Hope this helps.