Water

It cannot be stressed enough that water is the most important thing you consume. If you're trying to lose weight, or just be more active, you need water to help you. There is no magic amount of water that you should drink. The "8 glasses a day" rule is close and easy to remember, but not accurate if you're heavier than average, or more active.

A good way to figure out how much to drink is to take your weight, divide it in half and drink 80% of that figure in ounces of water every day. The other 20% of the water you consume will come from food. This figure is just a base; you'll need to add more for your activity. A good rule of thumb is 8 ounces for every 30 minutes of exercise. If you're fully hydrated before the activity, and you don't work out for too long or too hard, this should carry you. If you're working out harder or longer, you'll need more water and something with electrolytes.

There are many benefits to staying fully hydrated:

Water helps your body remove toxins in the most efficient manner, leaving the liver unburdened.
Water fills us up without adding any calories.
Water helps a person’s ability to exercise and burn calories.
Water is used for protein synthesis which is needed to build or repair muscles.
Water is a necessary ingredient in turning food and stored fat into energy. If you can't turn stored fat into energy, you can't lose weight.
Water consumption can reduce your risk of colon cancer.
Water consumption can reduce your risk of bladder cancer.

If something so simple can reduce your risk of cancer, there seems no reason to not have a glass of water.

A simple way to tell if you're fully hydrated is to make sure your urine is light yellow, never dark or amber in color.