Here's the code, embedded using Codebender!
​Before you proceed:
Remove the RX and TX cables from the Arduino board
Add your information in the code below by pressing the "Edit" button
#define SSID "name" // "SSID-WiFiname"
#define PASS "password" // "password"
​String msg = "GET /update?key=YOUR_WRITE_KEY"; //change it with your key...
Picture
Try downloading the Codebender plugin and clicking on the "Run on Arduino" button to program your Arduino board with this sketch. And that's it, you've programmed your Arduino board directly from your browser!
​
Now you can re-connect the RX and TX cables. Now check your ThingSpeak channel to see the first sensor's info!


/* 	Arduino IOT - Temperature (oC) and Humidity (%) on the web
	*Use the DHT-22 sensor to read temperature and humidity values
	*Send these values to www.thingSpeak.com with the ESP8266 serial Wifi module
	Dev: Michalis Vasilakis // Date:23/2/2016 // Update: 25/2/2015 // Ver. 1.3
	More info: http://www.ardumotive.com/iot-wifi-temp-and-humidity.html
	Tip: open the serial monitor for debugging 										*/

//Libraires
#include <stdlib.h>
#include <DHT.h>

/*------------------------DHT SENSOR------------------------*/
#define DHTPIN 2     		// DHT data pin connected to Arduino pin 2
#define DHTTYPE DHT22   	// DHT 22 (or AM2302)
DHT dht(DHTPIN, DHTTYPE);	// Initialize the DHT sensor
/*----------------------------------------------------------*/

/*-----------------ESP8266 Serial WiFi Module---------------*/
#define SSID "name"			// "SSID-WiFiname" 
#define PASS "password" 			// "password"
#define IP "184.106.153.149"// thingspeak.com ip
String msg = "GET /update?key=YOUR_WRITE_KEY"; //change it with your key...
/*-----------------------------------------------------------*/

//Variables
float temp;
int hum;
String tempC;
int error;
void setup()
{
  Serial.begin(9600); //or use default 115200.
  
  Serial.println("AT");
  delay(5000);
  if(Serial.find("OK")){
    connectWiFi();
  }
}

void loop(){
  //Read temperature and humidity values from DHT sensor:
  start: //label 
  error=0;
  temp = dht.readTemperature();
  hum = dht.readHumidity();
  char buffer[10];
  // there is a useful c function called dtostrf() which will convert a float to a char array 
  //so it can then be printed easily.  The format is: dtostrf(floatvar, StringLengthIncDecimalPoint, numVarsAfterDecimal, charbuf);
  tempC = dtostrf(temp, 4, 1, buffer); 

  updateTemp();
  //Resend if transmission is not completed 
  if (error==1){
  	goto start; //go to label "start"
  }
  
  delay(3600000); //Update every 1 hour
}

void updateTemp(){
  String cmd = "AT+CIPSTART=\"TCP\",\"";
  cmd += IP;
  cmd += "\",80";
  Serial.println(cmd);
  delay(2000);
  if(Serial.find("Error")){
    return;
  }
  cmd = msg ;
  cmd += "&field1=";   	//field 1 for temperature
  cmd += tempC;
  cmd += "&field2=";	//field 2 for humidity
  cmd += String(hum);
  cmd += "\r\n";
  Serial.print("AT+CIPSEND=");
  Serial.println(cmd.length());
  if(Serial.find(">")){
    Serial.print(cmd);
  }
  else{
    Serial.println("AT+CIPCLOSE");
    //Resend...
    error=1;
  }
}

 
boolean connectWiFi(){
  Serial.println("AT+CWMODE=1");
  delay(2000);
  String cmd="AT+CWJAP=\"";
  cmd+=SSID;
  cmd+="\",\"";
  cmd+=PASS;
  cmd+="\"";
  Serial.println(cmd);
  delay(5000);
  if(Serial.find("OK")){
    return true;
  }else{
    return false;
  }
}