How am I supposed to use GetComponent?

I hate to ask this question especially when it has been asked so many times, but I just don't get how get component works. All the other threads I've been to haven't helped in the slightest.

What I am trying to do is make an a fireball deal damage to another object when it collides with it. Usually I would use sendmessage, but here the player can charge up their fireball and make it do increased damage.

SO what I want to do is have the fireball directly take away the object's health based on it's charge level (if the object has a health variable).

 using UnityEngine;
 using System.Collections;
 
 public class FireBall_beh : MonoBehaviour
 {
     // Gameplay stuff
     public static float chargeLevel = 100f;
     public float speed = 12f;
     public float turnRate = 3f;
     public float lifeTime = 3f;
     private GameObject target;
     private Vector3 rayDir;
     
     // Charge level stuff
     private float particleStartSpeed;
     private float particleEmissionRate;
     private float particleRadius;
     public float baseDamage = 15f;
 
 
     void OnEnable ()
     {
         // change size depending on the charge level
         particleStartSpeed = particleSystem.startSpeed;
         particleEmissionRate = particleSystem.emissionRate;
         particleRadius = transform.localScale.y;
         
         particleSystem.startSpeed = (particleStartSpeed / 100) * chargeLevel;
         particleSystem.emissionRate = (particleEmissionRate / 100) * chargeLevel;
         transform.localScale = transform.localScale = new Vector3((particleRadius / 100) * chargeLevel, (particleRadius / 100) * chargeLevel, (particleRadius / 100) * chargeLevel);
         
         // Die after an ammount of time
         Invoke("Die", lifeTime);
                 
         // Do a sphere cast to see if there are players to lock onto
         RaycastHit hit;
         rayDir = transform.forward;
         if(Physics.SphereCast(transform.position, 10, rayDir, out hit, 20f))
         {
             //Debug.Log(hit.collider.name);
             
             if(hit.collider.tag == "Player")
             {
                 target = hit.collider.gameObject;
             }
         }
     }
 
 
     void FixedUpdate ()
     {
         // Move forward
         transform.Translate(0,0, speed * Time.deltaTime);
         
         // If this object has a target
         if(target != null)
         {
             // Check if the target is infront
             Vector3 targetDir = target.transform.position - transform.position;
             float angle = Vector3.Angle(targetDir, transform.forward);
             // Follow the target
             if(angle >= -90f && angle <= 90f )
             {
                 Vector3 lookTarget = target.transform.position - transform.position;
                 Quaternion rotation = Quaternion.LookRotation(lookTarget);
                 transform.rotation = Quaternion.Lerp(transform.rotation, rotation, Time.deltaTime * turnRate);
             }
         }
     }
     
     void  OnTriggerEnter(Collider other) 
     {
         // Calculate damage
         float damage = (baseDamage / 100) * chargeLevel;
         // Get the script of the other object and take away it's health (if it has a health variable)
     
         // Die
         Die();
 
     }
         
     void Die()
     {
         // Change particel system paramaters back to normal
         particleSystem.startSpeed = particleStartSpeed;
         particleSystem.emissionRate = particleEmissionRate;
         transform.localScale = new Vector3(particleRadius, particleRadius, particleRadius);
         // Clear target
         target = null;
         // Deactivate
         gameObject.SetActive(false);
         
     }
 }

The chargeLevel variable is a percentage 100 being how it is normally and it increases to 300% damage and size (this variable is set in the player script when the player fires it)

Thanks for all and any help and if there is a better way to do this please let me know.