Welcome in the wonderful world called Arduino ,
First use the # button if you want to publish code (you can modify your 1st post, select the code and press the #button)
The reading of the sensor will fail, so I rewrote your code a bit so it should work
#define THRESHOLD 750
int ledPin = 10; // Setup LedPin on Digital Pin 10
int sensorValue = analogRead(A0);
if (sensorValue < THRESHOLD )
digitalWrite(ledPin, HIGH); //High Reflectance.
digitalWrite(ledPin, LOW); // Low Reflectance.
To improve the stability of the readings, you can repeat them in a loop like in the function below.
unsigned int sharpRawAvg()
unsigned long raw = 0;
for (byte i=0; i<32; i++)
raw += analogRead(A0);
You can add this function to your code and call it from your sketch
replace int sensorValue = analogRead(A0);
withint sensorValue = sharpRawAvg()