This is why you need a bias resistor and how to calculate it.
CAUTION: This method will give you a highly nonlinear voltage output with temperature, because of both, the thermistor is a nonlinear device for which its resistance changes nonlinearly with temp and because of the voltage divider used for bias. If you don't know the transfer function of the thermistor with temperature you will be in deep trouble to read the temp and making the linearization. I suggest you use a device giving an already linear output with temp like the LM35 for instance.
Good luck.
