Monitoring voltage from a power supply

Ok I've been playing with this all afternoon and I was able to get a value of 1023 with 5v on Analog pin 0 and ground, cool!

I used the following code:

int Vin = 0;
int val = 0;

void setup() {
     Serial.begin(9600);
}

void loop() {
     val = analogRead(Vin);
     Serial.println(val);
     delay(1000);
}

As expected, I get 1023 every second with 5v supplied to pin 0. The next challenge is how to take this number and make it into a human readable voltage value? I know now that 1023 = 5v, so how would I convert that or a lower number?

I'm getting closer...