Brake fluid is what you use, fill the clutch reservoir. You will also need someone to help you to hold the clutch pedal to the floor while you bleed it. Under the hood ,driver side look where the transmission runs out toward the driver side. On top of the trans. there is a little brass nipple with a quarter inch nut on it, it is under some small hoses so make sure the engine is cool or you will burn your hand. Put a quarter inch wrench on that little nut while a friend holds the clutch pedal down. Rotate the wrench over the nipple from the cab toward the front of the car. After it squirts rotate it back closed, have friend raise the pedal. Repeat this process until you have a steady stream of fluid shooting out of the nipple with no air bubbles. Keep an eye on the fluid level in the reservoir, don't let it get to low! Just so you know, this is a temporary fix at best and is likely a problem resulting from an air leak, a bad master or slave cylinder or a worn or unseated clutch plate. Could possibly be a faulty automatic clutch adjusting mechanism. Good luck!