after try to find out how to measure the values of the thermistor 503 (that usually comes with the Arduino Starter Kit) I got it and I'm going to explain it to help others in my situation.
The 503 is a 50K@25C, so you need a R of 50 K to GND.
And the figures you need in order to "understand" the values sent by the thermistor:
float Ln_Resist =log(Resistance);
Temp = log(Resistance);
Temp = (1/(0.00237531+0.00024632*Ln_Resist+0.00000028*Ln_Resist3)-273);
return Temp; // Devolver temperatura