How to store words or sentence in a variable

I try to create a chat application for android by using a bluetooth conection!For that 2 smartphone 2 arduino AT mega 2560 and on every board i have a bluetooth device!My question is how i store word or sentence in a variable i’ve try with string but it doesnt work i get an error.So to communicate I send a message from my application by bluetooth to the 1st arduino board.Then from the first arduino board i sent it to the 2nd board by a serial communication and from the second board i sent it to the other phone by bluetooth.It only work if i sent number from the smarphone i can receive the same number on the other smartphone but it doesnt work when i sent word or setence.I also upload an image of my application you can download the picture.“Envoyer” means send in english and the message i receive should appear below the send bouton.Ty and sorry for my bad english

byte serialA; //comunication btw the phone and the arduino board
byte serialB; //comunication btw the 2 arduino board
void setup() 
{
 Serial1.begin(9600);  //connection btw the 2 board
 Serial.begin(9600);   //connection btw arduino and the bluetooth device
 } 
void loop() 
{
 serialA = Serial.read(); // read the value sented by the phone
serialB = Serial1.read(); //read the value sented by the arduino board to the 2nd arduino board
Serial1.write(serialA); //Sent to the 2nd arduino the value which i receive from the phone
Serial.write(serialB);  //Sent to the phone the value which i receive from the 2nd arduino board
}

You have to either use .available() to see if there is a character in the buffer before reading Serial OR you have to store the value in an 'int' and see if you received -1 which indicates that there were no characters in the buffer:

byte serialA; //comunication btw the phone and the arduino board
byte serialB; //comunication btw the 2 arduino board
void setup() {
 Serial1.begin(9600);  //connection btw the 2 board
 Serial.begin(9600);   //connection btw arduino and the bluetooth device
 } 
void loop() {
  if (Serial.available()) {
   serialA = Serial.read(); // read the value sented by the phone
   Serial1.write(serialA); //Sent to the 2nd arduino the value which i receive from the phone
  }
  if (Serial1.available()) {
    serialB = Serial1.read(); //read the value sented by the arduino board to the 2nd arduino board
    Serial.write(serialB);  //Sent to the phone the value which i receive from the 2nd arduino board
  }
}

OR

// NOTE: These must be 'int' to tell -1 from 'ÿ' 
int serialA; //comunication btw the phone and the arduino board
int serialB; //comunication btw the 2 arduino board
void setup() {
 Serial1.begin(9600);  //connection btw the 2 board
 Serial.begin(9600);   //connection btw arduino and the bluetooth device
 } 
void loop() {
   serialA = Serial.read(); // read the value sented by the phone
   serialB = Serial1.read(); //read the value sented by the arduino board to the 2nd arduino board

   if (serialA != -1)
      Serial1.write((char)serialA); //Sent to the 2nd arduino the value which i receive from the phone

    if (serialB != -1)
        Serial.write((char)serialB);  //Sent to the phone the value which i receive from the 2nd arduino board
}

Have a look at the examples in serial input basics. They store the incoming data in a char array which is the best way to store text on an Arduino.

Remember that the Arduino has very little memory and it would be very easy to use it storing sentences.

…R