#include <SoftwareSerial.h>

SoftwareSerial mySerial(2, 3); // RX, TX

#define DEBUG true

void setup() {
  // Open serial communications and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }
  mySerial.begin(9600);
  sendData("AT+RST\r\n", 2000, DEBUG); // rst
  sendData("AT+CWJAP=\"wifi namne\",\"password\"\r\n", 3000, DEBUG); //enter name and password
}

void loop() {
  int key;
  sendData("AT+CWMODE=1\r\n", 1000, DEBUG); //  access point
  sendData("AT+CIFSR\r\n", 1000, DEBUG); // get ip address
  sendData("AT+CIPMUX=1\r\n", 1000, DEBUG); // configure for multiple connections
  sendData("AT+CIPSTART=4,\"TCP\",\"api.thingspeak.com\",80\r\n", 1000, DEBUG); // turn on server on port 80
  key = 23;   //random number for testing
  String web = "GET http://api.thingspeak.com/update?api_key=KTQXXXXXXXXXXXXX&field1="";
  web += key;
  web += "HTTP / 1.0";
  web += "\r\n";
  web += "\r\n";
  String cipsend = "AT + CIPSEND = ";
  cipsend += 4;
  cipsend += ", ";
  cipsend += String(web.length());
  cipsend += "\r\n";
  sendData(cipsend, 1000, DEBUG);
  sendData(web, 10000, DEBUG);
  String closeCommand = "AT + CIPCLOSE = ";
  closeCommand += 4;// append connection id
  closeCommand += "\r\n";
  sendData(closeCommand, 3000, DEBUG);
}

String sendData(String command, const int timeout, boolean debug) {
  String response = "";
  mySerial.print(command); // send the read character to the esp8266
  long int time = millis();
  while ( (time + timeout) > millis()) {
    while (mySerial.available()) {
      // output to the serial window
      char c = mySerial.read(); // read the next character.
      response += c;
    }
  }
  if (debug) {
    Serial.print(response);
  }
  return response;
}

Source: esp8266 – AT+CIPSEND command does not return “>” in Arduino code but working manually – Stack Overflow

Source: Returning data from AT+CIPSTART from Arduino Uno

Source: Software serial : buffer size limit