To add a force in the direction of the object's own forward direction, you can use AddForce and use transform.forward as the vector direction. This vector (along with .right and .up) has a length of 1, so you can multiply it by any value you like to give the total power of the force applied. For example:

var power = 40.0;

function Update() {
    if (Input.GetButtonDown("Fire1")) {
        rigidbody.AddForce(transform.forward * power);
    }
}

To rotate an object using physics, you generally should use the rotational equivalent of AddForce, which is AddTorque. However, if you want to precisely rotate an object from -45 to 45 degrees, this might be better done without using the physics engine (since using physics forces tends to yield imprecise results).

I'm going to take a wild guess here and assume this is a power meter needle for a power gauge which goes from -45 to 45.

To implement this, I would have the power range go from 0-1 while you hold the fire button down, and then add the force when you release the button. The code might look something like this:

// public vars (settable in inspector)
var powerNeedle : Transform;
var maxPower = 40.0;
var powerMeterSpeed = 0.2;

// private vars (for internal use)
private var powerFactor = 0.0;


function Update() {

    if (Input.GetButtonDown("Fire1")) {
         // start the meter at 0 when the button is first pressed
         powerFactor = 0.0;
    }

    if (Input.GetButton("Fire1")) {
         // increase power factor smoothly while button is held down
         powerFactor += Time.time * powerMeterSpeed;
    }

    if (Input.GetButtonUp("Fire1") || powerFactor >= 1) {
        // apply force when button is released, or power reaches max
        rigidbody.AddForce(transform.forward * maxPower * powerFactor);
    }

    // set rotation of needle gameobject:
    var needleAngle = Mathf.Lerp( -45, 45, powerFactor );
    powerNeedle.rotation = Quaternion.Euler(0, 0, needleAngle);

}