Transforming from analog output to speed and acceleration with gy-61

Question

Maybe it's a simple question but I'm unable to get what I need.

My task is this: Get the acceleration and speed of each axis of the accelerometer.

I thought of something like:

const int VCCPin = A0;
const int xPin   = A1;
const int yPin   = A2;
const int zPin   = A3;
const int GNDPin = A4;

// variables
int old_x = 0;
int old_y = 0;
int old_z = 0;

int new_x = 0;
int new_y = 0;
int new_z = 0;

void setup() {
  // pin A0 (pin14) is VCC and pin A4 (pin18) in GND to activate the GY-61-module
  pinMode(A0, OUTPUT);
  pinMode(A4, OUTPUT);
  digitalWrite(14, HIGH);
  digitalWrite(18, LOW);

  // activating debugging for arduino UNO
  Serial.begin(9600);
} 

void loop() {
  new_x = analogRead(xPin);
  new_y = analogRead(yPin);
  new_z = analogRead(zPin);

  Serial.print("Difference in X=");
  Serial.println(abs(old_x - new_x));

  Serial.print("Difference in Y=");
  Serial.println(abs(old_y - new_y));

  Serial.print("Difference in Z=");
  Serial.println(abs(old_z - new_z));

  old_x = new_x;
  old_y = new_y;
  old_z = new_z;
} 

Unfortunately, the analogRead will give me a value between 0 and 1023 and I don't know how to convert this to speed and acceleration.

Any ideas on how to get this convertion?