hot to use raycast

how to return ray's position of intersection in the plane

Look at Plane.Raycast() and Ray.GetPoint(). (Note that it's pretty easy to find this sort of information in the documentation, and checking the documentation will usually be quite a bit faster than waiting for an answer on Unity Answers.)

You alse could perform the Raycast and use a RaycastHit object to get the informations. In the documentation they show this code

function Update () {
    var hit : RaycastHit;
    if (Physics.Raycast (transform.position, -Vector3.up, hit)) {
        var distanceToGround = hit.distance;

As you see you can use a RaycastHit object as an additional parameter. The member variables of RaycastHit can be found in the documentation, too. For example you could use

point The impact point in world space where the ray hit the collider.