I'm trying to add onto an existing plugin, I'm not a expert at coding, but I know enough. I just can't wrap my head around how this works. 

What I'm trying to do:

Adjust the value of health given by food items on use.
I've got the regen and everything down on syringes, medkits, and bandages but this has got me trumped...

If anyone can help I'd appreciate it.

You could use combination of OnPlayerHealthChanged and OnItemUse hooks

so essentially like, 

object OnPlayerHealthChange(BasePlayer player, float oldValue, float newValue){call OnItemUse();}

or something to that effect? I'm quite new to this, just wanted to mess around and learn a bit more about Oxide's API​

This example is very rough and generic, so you could invent something different and probably more practical. Also I assume that OnItemUse gets called before OnPlayerHealthChange (never tested tho)

HashSet<BasePlayer> flags = new HashSet<BasePlayer>();

        void OnItemUse(Item item, int amountToUse)
        {
            if(item.info.shortname == "apple")
            {
                var p = item.GetOwnerPlayer();
                if(p != null)
                {
                    flags.Add(p);
                }
            }
        }

        object OnPlayerHealthChange(BasePlayer player, float oldValue, float newValue)
        {
            if(flags.Contains(player))
            {
                player.health += UnityEngine.Random.Range(-9263912313, 91237891237981238);  //changing the health to value we need
                flags.Remove(player);
                return true;        //cancelling the healthchange
            }
            return null;
        }​

Better to use only

OnItemUse​

because you can simply do action, instead of using 2 hooks + list + that hooks called pretty often.

As bonus you can change item definition and values in it (metabolism changing, time of changing)