Im having trouble figuring out a way to make these two functions work.

Questions & Answers
1

I’m trying to create an apple projectile that will disappear if it hits a certain object. I understand that I am declaring apple as a gameobject and then trying to make it equal to a collider. I just need to know a way to make this logic work. Below are the two functions in question.

void Fire()
	{
		var apple = (GameObject)Instantiate (applePrefab, appleSpawn.position, appleSpawn.rotation);
		apple.GetComponent<Rigidbody> ().velocity = apple.transform.forward * 10;

		Destroy (apple, 5.0f);

		if (apple.gameObject.CompareTag("Enemy"))
		{
			FireCollision (apple);
		}
	}

	void FireCollision(Collider other)
	{
		other.gameObject.SetActive(false);
		health = health - 5;
	}
2

I don’t think you understand how collision is handled in Unity.
Each monobehaviour has several methods that unity tries to call through reflection.
One of these methods is: OnCollisionEnter(Collision collision)

These methods are called as soon as the monobehaviour collides with an object automatically.
You should create a script with following and put this on the applePrefab.

// Can be on the appleprefab aswel, as soon as the prefab  enters the game it will destroy after 5
void Start() 
{
     Destroy (gameObject, 5.0f);
}

// A script on the apple prefab should contain this method
void OnCollisionEnter(Collision collision)
{
     if (collision.CompareTag("Enemy"))
     {
          collision.gameObject.SetActive(false);
          // Health must be substracted from the player I suppose?
     }
}

Where your fire method will only contain:

 void Fire()
 {
     var apple = (GameObject)Instantiate (applePrefab, appleSpawn.position, appleSpawn.rotation);
     apple.GetComponent<Rigidbody> ().velocity = apple.transform.forward * 10;
 }