As per Spooler, you've probably got a Clutch Slave Cylinder (CSC) or Clutch Master Cylinder (CMC) or both going out. All too common problem with 370Zs.
Search this site for "CSC" and "CMC" for more info. Check the CMC level and condition of the fluid.
You might want to get a copy of the Factory Service Manual (FSM) if you plan on doing your own repairs. Nissan has had some copies taken down but you may still be able to find one.