One of the first things you do is to pump up the brakes, then hold it pushed, if it slowly goes down, it's the master cylinder leaking internally. If it is leaking anywhere you will see it.
Jack it up, pull off the wheels and see what you have. Pull off the rear drums, if there is any wetness at all around the wheel cylinders than it's leaking and needs to be replaced.
But the first thing you need to do is a visual inspection for leaks.