Code and connections instructions

#include <ESP8266WiFi.h>
#include <WiFiClient.h> 
#include <ESP8266WebServer.h>


#define IN_1  5          // L298N in1 motors Rightx          GPIO15(D1)
#define IN_2  4        // L298N in2 motors Right           GPIO13(D2)
#define IN_3  0           // L298N in3 motors Left            GPIO2(D3)
#define IN_4  2           // L298N in4 motors Left            GPIO0(D4)

String command;             //String to store app command state.

const char* ssid = "put name of your robocar here";
const char* pass = "put your password here";

ESP8266WebServer server(80);

void setup() {
 pinMode(IN_1, OUTPUT);
 pinMode(IN_2, OUTPUT);
 pinMode(IN_3, OUTPUT);
 pinMode(IN_4, OUTPUT); 
  Serial.begin(115200);

// Connecting WiFi

  WiFi.mode(WIFI_AP);
  WiFi.softAP(ssid, pass);
  IPAddress myIP = WiFi.softAPIP();
  Serial.print("AP IP address: ");
  Serial.println(myIP);

 // Starting WEB-server 

     server.on ( "/", HTTP_handleRoot );
     server.onNotFound ( HTTP_handleRoot );
     server.begin();    
}

void goAhead(){ 

      digitalWrite(IN_1, LOW);
      digitalWrite(IN_2, HIGH);
      digitalWrite(IN_3, LOW);
      digitalWrite(IN_4, HIGH);
  }



void goBack(){ 

      digitalWrite(IN_1, HIGH);
      digitalWrite(IN_2, LOW);
      digitalWrite(IN_3, HIGH);
      digitalWrite(IN_4, LOW);
  }

void goRight(){ 

      digitalWrite(IN_1, HIGH);
      digitalWrite(IN_2, LOW);
      digitalWrite(IN_3, LOW);
      digitalWrite(IN_4, HIGH);
  }

void goLeft(){

      digitalWrite(IN_1, LOW);
      digitalWrite(IN_2, HIGH);
      digitalWrite(IN_3, HIGH);
      digitalWrite(IN_4, LOW);
  }

void goAheadRight(){

      digitalWrite(IN_1, LOW);
      digitalWrite(IN_2, HIGH);
      digitalWrite(IN_3, LOW);
      digitalWrite(IN_4, HIGH);
   }

void goAheadLeft(){

      digitalWrite(IN_1, LOW);
      digitalWrite(IN_2, HIGH);
      digitalWrite(IN_3, LOW);
      digitalWrite(IN_4, HIGH);
  }

void goBackRight(){ 

      digitalWrite(IN_1, HIGH);
      digitalWrite(IN_2, LOW);
      digitalWrite(IN_3, HIGH);
      digitalWrite(IN_4, LOW);
  }


void goBackLeft(){ 

      digitalWrite(IN_1, HIGH);
      digitalWrite(IN_2, LOW);
      digitalWrite(IN_3, HIGH);
      digitalWrite(IN_4, LOW);
  }


void stopRobot(){  

      digitalWrite(IN_1, LOW);
      digitalWrite(IN_2, LOW);
      digitalWrite(IN_3, LOW);
      digitalWrite(IN_4, LOW);

 }



void loop() {

    server.handleClient();

    

      command = server.arg("State");
      if (command == "F") goAhead();
      else if (command == "B") goBack();
      else if (command == "L") goLeft();
      else if (command == "R") goRight();
      else if (command == "I") goAheadRight();
      else if (command == "G") goAheadLeft();
      else if (command == "J") goBackRight();
      else if (command == "H") goBackLeft();
      else if (command == "S") stopRobot();

}



void HTTP_handleRoot(void) {
if( server.hasArg("State") ){
       
          Serial.println(server.arg("State"));
  }
  server.send ( 200, "text/html", "" );
  delay(1);

}

download the app from here