loop() is already a loop, you don't need another loop "while (Serial.available() > 0)", just use something like this:
if ( Serial.available () > 0 )
{
static char input[16];
static uint8_t i;
char c = Serial.read();
if ( c != '\r' && i < 15 ) // assuming "Carriage Return" is chosen in the Serial monitor as the line ending character
input[i++] = c;
else
{
input[i] = '\0';
i = 0;
//do whatever with input string...
Serial.println( input );
}
}