Need help with procedures and variables

So. Im trying to create an item called the totem of posidon, which will give the player waterbreathing when its in the inventory. i want it to be dropped by zombies with a random chance. is there any way to do this? ive created the item, but i dont know how to get zombies to drop it, how to give the player its effect when in the inventory, or where to find the events to make this happen. i know a little bit about coding and ive made phone apps before using a similar creator, so i know the basics of variables and if/then stuff, i just cant find the triggering events.