How can i shoot with the spacebar

Alright, it would help if you posted up your script. You really shouldn't ask people to write scripts for you, or even put them up for you. The is to help with your problem, not give you code. What Cristian Stewart said was correct, but obviously that's not the whole thing. This will probably be the only time someone will help you by giving you a script, but I'm going to explain things to you:

First, use the default code that's already set up:

 function Update (){
 }

The Update function is called twice a frame. Next we have to add the variables that you would need:

 var fireBall : GameObject;
 var shootPoint : Transform;

 function Update (){
     }

"fireBall" needs to be a GameObject. "shootPoint" needs to be a transform because that's where the fireBall is going to come out. If you want it to come out of his mouth, put an empty gameObject near his mouth. Make sure the shootPoint is a child of the object that has the script. Now for the actual code that makes it happen:

 var fireBall : GameObject;
 var shootPoint : Transform;
 
  function Update (){
         if(Input.GetKeyDown("Fire1")) || Input.GetKeyDown("space")){
 Instantiate(fireBall, shootPoint.position, tranform.rotation);
      }

The first line of code, the "if" statement, means that whatever below will only be executed if anything in the if statement is met. The Input.GetKeyDown("Fire1")), means if the input: "Fire1" (Left Mouse Click) is down, it will execute the code.

 if(Input.GetKeyDown("Fire1"))

The completed line is above. Next, we want it to shoot if they click, OR if they hit "space". To do that we need to put in the rest of the if statement:

 if(Input.GetKeyDown("space"))

To make sure it means OR, we need these symbols ||. || means OR. So the completed line is:

 if(Input.GetKeyDown("Fire1")) || Input.GetKeyDown("space"))

To keep the script organized, and to make sure the if statement doesn't mess with any other code, we put in brackets:

 if(Input.GetKeyDown("Fire1")) || Input.GetKeyDown("space")){
 }

Now the conditions are set we need it to actually DO something. For that we INSTANTIATE something. Instantiate means "create" after we write Instantiate, we need to write what's instantiate, and how it's going to be displayed. So the first part inside the parenthesis is what's going to be instantiated, in this case, the fireBall:

 Instantiate(fireBall, xxxxx,xxxxx);

Next, we need to display where it's going to be shot from. Normally, it's written like this:

 Instantiate(fireBall, transform.position, xxxxxxx)

With that, it will be shot from the center of the object it's attached to. But to make it come from a specific place we write it like this:

 Instantiate(fireBall, shootPoint.position, xxxxxxx)

Now we need to make sure it's rotation is displayed too:

 Instantiate(fireBall, shootPoint.position, tranform.rotation)

With the above code, the fireball will have no rotation, which is fine.

So that's how it works. I really hoped this helped. The completed script is this:

 var fireBall : GameObject;
 var shootPoint : Transform;
 
  function Update (){
         if(Input.GetKeyDown("Fire1")) || Input.GetKeyDown("space")){
 Instantiate(fireBall, shootPoint.position, tranform.rotation);
      }

 if (Input.Get$$anonymous$$eyDown("Fire1") || Input.Get$$anonymous$$eyDown("space"))