CODE EXAMPLE FOR CSHARP

oncollisionenter unity

void OnCollisionEnter(Collision collision) {
       if (collision.gameObject.tag == "Door")
       {
               // DoorScript is the name you gave to the script on the door
               DoorScript script = collision.gameObject.GetComponent<DoorScript>();
               // OpenDoor is a method in your door object's script
               script.OpenDoor();
       }
}