If store -bought it's probably just the nut with dried meat inside (no juice), so go ahead and whack it anywhere with a hammer / icepick, anything that works.
If fresh -- with the husk still on and juice inside -- first keep it upright then hack at the top with a machete or other heavy blade all around the top until the nut is exposed. Using the tip of the machete (or a hammer and icepick) open the nut at the top just enough to drink from, or to pour out the juice.
Islanders will typically drive a sharpened stake into the ground at an angle, then fast pitch the coconut at the stake, splitting it open. Safer, but takes practice and space.
You can also buy coconut juice and coconut milk in cans.
Good luck and be careful with that machete :)
Source(s):
travel in southeat asian and micronesia, living in Japan