So get two arduinos, connect Rx on one to Tx on the other, and vice versa for the second part, and connect the grounds.
Start with a simple sketch and go from there. Use the Reference page and the Learning page
to understand what is being done.
byte LEDpin 13;
byte inputdata;
void setup()
{
pinMode (LEDpin, OUTPUT);
Serial.begin(9600);
}
void loop()
{
Serial.write (1);
if (Serial.available()>0)
{
inputdata = Serial.read();
digitalWrite (LEDpin, HIGH);
delay (250);
digitalWrite (LEDpin, LOW);
}
} // end loop