People like to use warm water at 30 to 50 degree rather than boiling water, so what if I use a temperature sensor to control the temperature of the water? What should the code be？ I am a beginner and really not familiar with the function of temperature sensor or heating unit, how to programme? Thank you for your help.

You need to create a PID controller. That has a setpoint for the desired temperature, a measured value for the actual temperature and an output that controls the temperature.