Firstly, to get the lovely coconut water out (very good for you):
You will see the coconut has 3 eyes on one end. One of these will be much softer than the other two. Simply try pushing a screwdriver in - when you get the right one it will yield fairly easily.
As the hole made isn't huge, and there isn't another hole to let air in, it will trickle out quite slowly when you upend the coconut. I don't have a great deal of patience so I balance it on a glass and leave it to get on with it until it stops dripping.
Next to crack it open. If you consider the eyes of the coconut to be the top then it has a weak seam around its middle. However, when I say weak, it still needs a bit of force to open it. Place the coconut in a plastic bag, take it outside, and knock it firmly onto the edge of a step or wall, keeping the eyes at the top and knocking the middle.
It may take a couple of firm whacks but it should break fairly cleanly in two.
And voila, lovely fresh coconut ready to enjoy.