Before you can brew many potions in Minecraft, you'll need to know how to make a fermented spider eye.
MINECRAFT | How to Make Fermented Spider Eyes! 1.15.2
Here's how to gather everything you need to make a fermented spider eye:
Find a brown mushroom. They grow in dark areas, or you can get them by mining giant brown mushrooms.
Craft Sugar. All you need is sugar cane, which grows in stalls near water.