How to use 503 Thermistors for temperature?
First of, 503 means 50 x 10^3, ie 50K Ohm: that is the maximum resistance of this thermistor (the higher resistance occurs for lower temperatures). However, having just that without any precise reference may prove difficult to find a relevant datasheet that could help you calculate the temperature from the measured voltage.

I think what could work well is just simple calibration by establishing a mapping between
the value returned by analogRead() and the temperature measured by a real, good precision, thermometer.

Establighing the mapping for 2 extreme temperatures would provide a first approximation on computing any temperature from the read value.

However, such an approximation would not be very accurate because thermistors are not linear, ie their resitance don't vary linearly with temperature.

What you can try to do then, is to increase the number of mapped value&lt;->temperature with regular intervals of temperatures; then you can use linear approximation between two consecutive mapped values, which would provide better results.

The difficulty here is to "produce" the temperatures for which you want to find the mapped value: if you can spend one year for that, then just let the chqanging seasons operate that for you :-) otherwise use what you can find at home: fridge, oven, fan, heater...