Example of common approach to use Arduino Serial

classic_serial.cpp

// If we have received data
if (Serial.available() > 0)
{
    // Read the order sent by the computer
    order_received = Serial.read();
    // If the received byte is the character 'a'
    if (order_received == 'a')
        // This correspond to the action GO_FORWARD
        // It will be used later to send speed order to the motor
        action = GO_FORWARD;
}