Input.GetButtonUp

Description

Returns true the first frame the user releases 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 pressed the button and released it again.

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

using UnityEngine;
using System.Collections;

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