Read analog voltage with

Hi

I bougth this one year ago:

Today, i'm triying to read one analog voltaje, conecting the 3.3V pin or 5V pin to another

this is my code, it's very simple, but it doent work

 

int sensor = 5; //pin 5, el PE4
float voltaje = 0;

void setup() {

//declaro que el pin de la entrada analogica será de lectura
pinMode(sensor, INPUT);
Serial.begin(9600); //opens serial port, sets data rate to 9600 bps
}

void loop() {

voltaje = analogRead(sensor);

Serial.println (voltaje);
delay(2000);
}

What can I do to read the real analog voltage? Because the "Serial Monitor" in energía doesnt write me the correct value of the voltage

thank you and sorry for my English