Not drinking enough water can have a major effect on your performance. You should start any exercise session well hydrated. This means drinking water regularly throughout the day.
The choice of drink depends on the intensity and duration of the exercise, and your training goals.
In general:
Only water is needed for moderate exercise that lasts less than an hour. An isotonic sports drink, milk, or a combination of high-carbohydrate food and water for hard sessions that last longer than an hour.
You can make a homemade sports drink with 200ml of squash (not low calorie), 800ml water and a large pinch of salt.