Your body needs water to function well. Ideally, you should drink 2 litres of water a day (6 to 8 glasses of liquids), to keep your body hydrated. If you do a lot of physical activity, increase your water.
If you don't drink enough fluids, you may become dehydrated which can leave you feeling tired, make you constipated and cause headaches.
One way is to check the colour of your urine. If it's a pale straw colour you are fine - any darker and it might be a good idea to drink more water.
Water is the ideal beverage to drink. Bottled or sparkling water are good, but tap water is fine too. If you find it difficult to drink a lot of water, other beverages can contribute to your daily fluid intake.
Tea can significantly contribute towards your daily fluid intake.