Activating a relay when a temperature exceeds a certain value

Hi everyone,

I’m very new to programming, and I would like to try building a A/C control unit
I have a working temperature sensor, and I have a relay.

How do I program the arduino to “start” the relay?

Here is the code I’ve got so far:

 * name:LM35 Temperature Sensor
 * function:
 * LM35 output voltage has a linear relation with the Celsius temperature, output of 0 v when 0 ℃, 
 * every increase 1 ℃, the output voltage increase 10 mv
//lm35 attach to A0
const int relayPin =7; //the "s" of relay module attach to
#define lmPin A0  //LM35 attach to
#include <Wire.h> 
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27,16,2);  // set the LCD address to 0x27 for a 16 chars and 2 line display

float tem = 0;
long lmVal = 0;

void setup()
  lcd.init();  //initialize the lcd
  lcd.backlight();  //open the backlight 
  pinMode(relayPin, OUTPUT); //initialize relay as an output
void loop()
  lmVal = analogRead(lmPin);//read the value of A0
  tem = (lmVal * 0.0048828125 * 100);//5/1024=0.0048828125;1000/10=100
  lcd.setCursor(5,0);//place the cursor on 5 column,0 row
  lcd.setCursor(0,1);//place the cursor on 0 column,1 row
  lcd.print("Tem= ");//print"Tem= "
  lcd.setCursor(5,1);//place the cursor on 5 column,1 row
  lcd.print(tem);//print tem
  lcd.print(char(223));//print the unit" ℃ "
  delay(200);//delay 200ms

You would use

digitalWrite(relayPin, HIGH);