undefined index :username in $_GET()

Question

    define('HOST','localhost');
    define('USER','id3000922_siddhi');
    define('PASS','siddhi123');
define('DB','id3000922_123  ');
    $con = mysqli_connect(HOST,USER,PASS,DB) or die('Unable to Connect');
    $username = $_GET['username'];

$password = $_GET['password'];

$email = $_GET['email'];
    if($username == '' || $password == '' || $email == '')
    {

    echo 'please fill all values';
    }
    else{
    $sql = "SELECT * FROM signup WHERE username='$username' OR email='$email'";
        $check = mysqli_fetch_array(mysqli_query($con,$sql));
    if(isset($check)){
    echo 'username or email already exist';
    }else{
    $sql = "INSERT INTO signup (username,password,email) VALUES('$username','$password','$email')";
    if(mysqli_query($con,$sql)){
        echo 'successfully registered';

   }
    else{

        echo 'oops! Please try again!';

    }
}

        mysqli_close($con);
    }

in the above code it is showing:

Notice: Undefined variable: username in /storage/ssd4/922/3000922/public_html/User_registration/Register.php on line 8

Notice: Undefined index: password in /storage/ssd4/922/3000922/public_html/User_registration/Register.php on line 10

Notice: Undefined index: email in /storage/ssd4/922/3000922/public_html/User_registration/Register.php on line 12 please fill all values