There are a few things you can do to try and figure it out..............
Get a subaru Repair manual, and go for it. The only way I learned was to do it, there are a couple of things to check, thermostat, look for leaks, radiator cap, radiator, Water pump, Water level, make sure the air bubble is out , make sure you fan is coming on, or it might be a blown Head Gasket, If anything you might get to know your car a little bit, witch would never hurt..................
Jeff