This will show how to use analogRead to get a voltage from an circuit.  This circuit is a voltage divider.   I have two 1000 Ω resistors in series.  It will cut the voltage in half.  Without a multimeter it will show you have to display the analog (floating point) voltage.  A5 is the analog port that will read the value of the voltage divider.  EE: Resistor page shows you how resistors in circuits are calcuated. 

analogWrite is 0 to 255 (8 bit).  analogRead is 0 to 1024 ( 10 bit). So to convert analogRead into a voltage.

(5. / 1023. ) *analogPort value.

I will put the below code into the Arduino and copy the ino file up after building the circuit. 

V_R2 = (5./1023.) * readVal; 
The dot after the numbers make the math floating point. 

#include <Wire.h>

float V_R2 = 0;
int readVal = 0;
int readPin = A0;
int delayTime = 1000;

void setup() {
  Wire.begin(0x6);                // join i2c bus with address #6
  Wire.onReceive(receiveEvent); // register event  
  pinMode(readPin, INPUT);

void loop() {
readVal = analogRead(readPin);
V_R2 = (5./1023.) * readVal;


// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(int howMany) {

4.97V on 5V rail. 
Multimeter: 2.476V
Com9 Serial Mon: 2.46V

The 4.97V voltage divider of two 1kΩ resistors would be 2.485V
R1: 1.002 kΩ
R2: 993Ω
Rtotal: 1.995kΩ