How to generate a different number every time?

I am making a connect 4 game. I made an array that stores all the x-positions for the disks to fall in. I want the computer to generate a random x-position for the disk to fall in every time. Here is my code.

`
#pragma strict

// Disks that we’re playing with
public var RedDisk : GameObject;

// Converts array of numbers into a variable
private var choices : float = [-26.92, -17.8, -9.2, 0, 9.2, 17.8, 26.92];
private var howManyThings : float = choices.length;
private var myRandomIndex : float;
myRandomIndex = Random.Range (0, howManyThings);
private var result : float;

private var PlayerTurn : boolean = false;

// Happens every frame
function Update () {

// If it is the enemy’s turn,
if (PlayerTurn == false) {

  result = choices[myRandomIndex];

  // Create a red disk in a random spot
  Instantiate (RedDisk, new Vector3 (result, 10, 0), Quaternion.identity);

 // User can now put his disk
PlayerTurn = true;

}

}
`

Create the random number every time you need it. Not just once when the app starts.

myRandomIndex = Random.Range (0, howManyThings); 
result = choices[myRandomIndex];