Use this Hydration Calculator to find out.
If you are exercising, you need to drink enough fluids to help metabolize fat and keep you from retaining water. For someone just beginning, the average is a 1/2 ounce of water per pound of weight. ie: A 120 pound person would need 60 ounces of fluids per day.
Keep in mind that milk, broth, tea, etc counts too. Just watch the amount of sugar in the beverages you choose.
Now, go grab an ice cold glass of water!