1999 Ford Ranger clutch pedal very spongy

When I press the cluch down it doesnt engage untill I press it to the floor. It is getting worse and sometimes I cant put it into 2nd ear or first untillI stop the truck and turn the engine off.I understand that there is a master cylinder and that this is a hydrolic system is there a place where you fill this resevore up? Or is this a clutch problem