Two Arduinos: One master controlling the other slave.

Hi All, Can anybody point me to links/documentation? How I can make two Arduinos communicating each other. One Master Arduino is sending instructions to slave Arduino. Thanking you in advance. BR

Connect Master Tx to Slave Rx,
Master Rx to Slave Tx.

Simplest implementation:

//Master:
Serial.print("A");

//Slave:
if (Serial.available()>0){
incomingByte = Serial.read();
  if (incomingByte == 'A'){ 
  // do something  
  }
}

If you're looking to do it over wifi, try using Temboo with Amazon SQS. Works brilliantly and isn't too hard to set up.

https://temboo.com/arduino/message-passing