How much water should you drink in a day? I suffer with headaches and find water helps but I'm always dehydrated and thirsty yet my palms are sweaty

Get checked! Being constantly thirsty is not normal in most cases. You really need to get checked to see what's causing this. Making sure you don't have diabetes or thyroid problems is important to do! There are other causes, as well, but only getting an evaluation by a doc can sort this out. Then treatment can begin!