You're trying to make a calibration curve for your thermocouple, so you should know the temperature at two calibration points. Did your lab test the voltage at two known temperatures? Often, students measure the voltage of a thermocouple when it is inserted into boiling water and melting water (mixed water and ice).
If you know the voltage and temperature for two points, you can figure out how much the voltage changes when the temperature changes (the sensitivity). For example, if I read 10.5 volts when the thermocouple is in the freezing water and 0.5 volts when the thermocouple is in the hot water, I know that the voltage decreases 10 V over 100°C, meaning that the voltage changes by -0.1V/°C. Now, if I measure 5.5 V from the thermocouple, I can relate that to 50°C.