Did you know that you body is made up of about 70% water? You can imagine we need to be drinking water every day to stay hydrated. My general rule for the amount of water to drink is 1/2 of your body weight in ounces. If you weight 120 Ibs then you need to drink about 60 ounces (about a gallon). If you weigh more you need more water. If you weigh less then you don't need as much water.
The secret to good hydration is to drink water before you get thirsty. Once you start feeling thirsty you are already on you way to being slightly dehydrated.