Help with AI

Hello everyone, since I started back at college I have been working on my FMP. Up to now its going well I have setup quite a lot. You can see what I have done already here: http://www.youtube.com/channel/UCrOw2DA0dlAy4oMwIEzFsUA/

I've gotten to the point where I need to have my AI avoid buildings, walls, high terrain.

I've been looking at using raycasts to do this but I have no idea where to start.

My current AI

 //var for health and setting the amount
 //static var health : float = 100.0;
 //players transform
 var player : Transform;
 var playerGO : GameObject;
 //default speed of the enemy
 var speed = 5.0;
 //how far the enemy will attack from
 var chaseRange = 50.0;
 //attacking range
 var dieRange = 5.0;
 //last time enemy attacked
 var lastShotFired = 0;
 //how often the enemy can attack
 var attackTime = 2;
 //setting up var for the enemies controller(Makes it walk)
 private var controller : CharacterController;
 var bulletTraceGeneratorScript : GameObject;
 
 function Start()
 {
 playerGO = GameObject.FindWithTag("Player");
 player = playerGO.transform;
 
 bulletTraceGeneratorScript = GameObject.FindWithTag("BulletTrace");
 //gets the controller
     controller = GetComponent(CharacterController);
 //    yield MyWaitFunction (2.0); (OLD CODE)
 }
 
     
 function Update()
 {
 //if the players var is empty put nothing in there
     if (player == null)
         return;
 //How far away from the player the enemy is
     var range = Vector3.Distance(player.position, transform.position);
 //If the enemy is withtin attacking range(attack)
     if (range <= dieRange)
     {
 //if the time is 2(attacktime) past the lastshotfired then attack
    if (Time.time > lastShotFired + attackTime) {
     
     Debug.Log("Attacking");
     animation.Play("attack");
     animation["attack"].speed = 2;
     animation["attack"].wrapMode = WrapMode.Loop;
     lastShotFired = Time.time;
     player.GetComponent("health").health -= 10;
 
     }
     
 
 //    yield return new WaitForSeconds(attackTime);(OLDCODE)
 
     }
 //if it isnt in the attacking range
     else if (range >= chaseRange)
     {
     //enenmy looks at the player
         transform.LookAt(player);
     //setting up the var so the enemy knows which way to go
         var moveDirection : Vector3 = transform.TransformDirection(Vector3.forward);
     //moving the enemy toward the player (time.deltaTime*speed - making sure it is moving correctly)
 
        controller.Move(moveDirection * Time.deltaTime * speed);
      //this checks to see if the enemy is hovering above the ground
      //this is common in most AI as it only moves the enemy straight towards the player
      //this however fires a raycast at the floor which then adjusts the enemies position to
      //the hit point of the raycast
      
        //setting up the raycast var   
     var hit : RaycastHit;
     //if the raycast (from the enemy (-up) downwards) hits something below then it will
     //put the value in the hit var
     if (Physics.Raycast (this.transform.position, -Vector3.up, hit)) {
        //this is the distance from the hit point and the enemy
        distanceToGround = hit.transform.position;
        
        //DEBUG STUFF
        //print("Distance: "+distanceToGround);
        //print("Hitting "+hit.transform);
        
        //the commented line below draws a line in the editor so that I can check it is working
        //Debug.DrawLine (this.transform.position, hit.point, Color.red, 1);
        
        //this moved the enemies position from where it was(floating) to the floor
        this.transform.position = hit.point;
        
     }
 
     animation["RunninInPlace"].speed = 1;
     animation.Play("RunninInPlace");
         animation["RunninInPlace"].wrapMode = WrapMode.Loop;
         
     }
     //if it isnt doing any of the above stand still and play the idle animation
     else
     {
     //plays the idle animation
     animation.Play("idle");
     //loops the animation
     animation["idle"].wrapMode = WrapMode.Loop;
     }
 }

If anyone has any idea how I could implement raycasts to make my AI avoid stuff please help. I'm really stuck with this.

I don't want anyone to do this for me. Just give me a way of doing it.

Thanks for any help you can give :)

Mike