There are many techniques that will do just fine in a case like this, but here is one that you can use if you don’t have a hammer handy.
Step 1: Locate the eyes of the coconut (three dark spots on one end) and grab a screwdriver, corkscrew or sharp scissors and poke it into one of these eyes. The coconut water will drain out of the coconut. If you are having trouble repeat with a second eye and it will drain faster.
Step 2: Grab in your dominant hand a butcher’s knife and hold the coconut with the other. Make a mark as to where you are going to start cutting and hit the coconut hard with the sharp end of the knife.
Step 3: Repeat this process until it cuts through the shell. Rotate the coconut so that the shell opens completely in a perfect circle.
If you don’t feel confident and you don’t have a butcher’s knife handy, just grab a screwdriver and a hammer. After you do step 1 insert the screwdriver into one of its eyes and hit it hard with the hammer. Rotate the coconut as needed until it cracks open.