You can find out the world position of your mouse using Camera.mainCamera.ScreenToWorldPoint(Input.mousePosition). You will have to set the z component of Input.mousePosition by yourself. To do this you can either use the distance of the cam to your ground, or if this is not fixed (e.g. terrain with different heights) you can do a raycast from the mouse position to find out the distance. Then you would do something like this (pseudo code):

LineRenderer lr;

void Update () {
  if (Input.GetButtonDown("Fire1")) {
    lr.SetPosition(0,GetWorldPoint());
    lr.SetPosition(1,GetWorldPoint());
    lr.enabled = true;
  }

  if (Input.GetButton("Fire1")) {
    lr.SetPosition(1,GetWorldPoint);
  }

  if (Input.GetButtonUp("Fire1")) {
    lr.enabled = false;
  }
}