This is how my grandmother taught me to open a pomegranate: put on one of my grandfather's old work undershirts and have at it!
This is how I open a pomegranate as a less-messy but almost certainly less-fun adult: with a small knife, slice off the crown and then score the tough outer skin of pomegranate in quarters. Try to puncture as few seeds - more properly, arils - as possible; you will puncture some, but don't worry about it too much. You can try to make the scores in curves but while it is then easier to open it is more complicated to cut in the first place and can be frustrating. Fill a large bowl with water. In fact, it's a good idea to fill the bowl with water before you start cutting the fruit. Submerge the scored pomegranate and - gently! - underwater, pull the quarters apart.
You can flip the quarters inside out and tap the seeds out that way, but sometimes the skin is too thick and you crush more seeds than you would if you just push them out gently and rub them with your fingers and thumbs. The papery insides will float to the top where you can skim them away. Drain and pick through the seeds to make sure there are no inner rinds and membranes attached. Leave draining above a bowl or spread out on a plate or baking sheet to dry for an hour, then refrigerate if saving for later. Otherwise, have at it!