Input.GetButtonDown

Description

Returns true during the frame the user pressed down the virtual button identified by buttonName.

You need to call this function from the Update function, since the state gets reset each frame. It will not return true until the user has released the key and pressed it again.

Use this only when implementing action like events IE: shooting a weapon.
Use Input.GetAxis for any kind of movement behaviour.

#pragma strict
public var projectile: GameObject;
function Update() {
	if (Input.GetButtonDown("Fire1"))
		Instantiate(projectile, transform.position, transform.rotation);
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public GameObject projectile;
    void Update() {
        if (Input.GetButtonDown("Fire1"))
            Instantiate(projectile, transform.position, transform.rotation);
    }
}