Converting to degrees celsius for a thermistor

I have this program but the readings i get are not in degrees celsius. What conversion do i have to put in to convert the temperature i get from the thermistor to degrees celsius

/*This program was borrowed from makezine.com and edited by Patrick B. It is designed to protect a plant and inform the user
when the plant requires attention by shining a specific color from the rgb led as well as displaying the data on a lcd screen.*/

// I am using the LCD Library in this program
#include <LiquidCrystal.h>

//setting up the pins to which the different sensors get connected to
int moistPin = 2;
int tempPin = 0;
int lightPin = 1;

//initialize variables to store readings from sensors
int moistVal = 0;
int tempVal = 0;
int lightVal = 0;

//setting up pins for the rgb led
int redPin = 6;
int greenPin = 5;
int bluePin = 4;


// setting up the pins to which the lcd is connected to
LiquidCrystal lcd(7,8,9,10,11,12);

void setup()
{
  //displaying the data on a serial monitor
  Serial.begin(9600);
  
  //setting the LED pins to output mode
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);
  
  //setting the LED pins to on
  digitalWrite(redPin, HIGH);
  digitalWrite(greenPin, HIGH);
  digitalWrite(bluePin, HIGH);

  // setting up the LCD's number of columns and rows: 
  lcd.begin(16, 2);
}

void loop()
{
  moistVal = analogRead(moistPin);
  
  //display moisture reading on lcd
  lcd.clear();
  lcd.print("moisture ");
  lcd.print(moistVal);
  
  //turn on LED to blue, others off if moisture pins are touching together
  if (moistVal > 1000)
  {
    blue();
  }
  else 
  { 
    off();
  }
 
  //delay 4 seconds (wait)
  delay(4000);
   //display moisture reading on lcd
   
  tempVal = analogRead(tempPin);
  lcd.clear();
  lcd.print("temperature ");
  lcd.print(tempVal);
  
  if (tempVal > 148)
  {
    red();
  }
  else 
  { 
    off();
  }
  
  delay(4000);
  
  lightVal = analogRead(lightPin);
   //display the moisture reading from soil probes on lcd
  lcd.clear();
  lcd.print("light ");
  lcd.print(lightVal);
  
  // if light sensor reads 600 or brighter turn on the green colour of the rgb led
  if (lightVal > 600)
  {
    green();
  }
  else 
  { 
    off();
  }

  delay(4000);
 
}

void blue()
{
  digitalWrite(redPin, HIGH);
  digitalWrite(greenPin, HIGH);
  digitalWrite(bluePin, LOW);
}

void red()
{
  digitalWrite(redPin, LOW);
  digitalWrite(greenPin, HIGH);
  digitalWrite(bluePin, HIGH);
}

void green()
{
  digitalWrite(redPin, HIGH);
  digitalWrite(greenPin, LOW);
  digitalWrite(bluePin, HIGH);
}

void off()
{
  digitalWrite(redPin, HIGH);
  digitalWrite(greenPin, HIGH);
  digitalWrite(bluePin, HIGH);
  
}

void on()
{
  digitalWrite(redPin, LOW);
  digitalWrite(greenPin, LOW);
  digitalWrite(bluePin, LOW);
}

What conversion do i have to put in to convert the temperature i get from the thermistor to degrees celsius

The one specified for the sensor, which relates Kelvin to millivolts.