Keep a temporary variable to act as your final speed, and a constant that is your normal speed, along with another variable to act as a multiplier. A simple example:

var NaturalSpeed = 10;
var tempSpeed : float;
var SpeedMultiplier = 1.2; // for 20% speed increase while sprinting

var moveDirection : Vector3; //this determines which direction we should be moving towards

function Update()
{
    moveDirection = Vector3.zero; //this rests our move direction to nothing so we wouldn't move unless movement keys were pressed
    tempSpeed = NaturalSpeed; //this rests our speed back to natural speed

    //this is just a simple movement check for demonstrating sprint; use your own methods of course
    moveDirection.x += Input.GetAxis("Horizontal");
    moveDirection.z += Input.GetAxis("Vertical");

    if(Input.GetKey(KeyCode.LeftShift)) //this checks to see if the player is holding left shift
    {
       tempSpeed *= SpeedMultiplier; //this increases our speed by the multiplier
    }

    transform.Translate(moveDirection.normalized * tempSpeed * Time.deltaTime); //this moves our player to the correct direction at the determined speed (sprinted or not), smoothed over time
}

Take a course on scripting maybe?