The main goal of alchemy is to craft powerful potions and poisons that can have devastating effects in combat. To successfully craft potions or poisons in the Elder Scrolls Online, you need to farm reagents (plants, monster remains) and solvents (water, oils, etc.).
Each reagent (flower) has four different traits. You can find a full list of all reagent traits in the Alchemy Reagent Traits Guide. Here we have a Columbine and a Corn Flower with various traits. At first, the traits will be listed as "unknown". When you successfully mix potions or poisons, the traits will be discovered. You can find the most efficient way to do that in the Alchemy Traits Discovery Guide.
The same procedure works with poisons. However, instead of selecting a pure water we want to select a viscous oil such as the Alkahest in the picture. Already mentioned above, the more reagents you mix together to create a poison, the shorter the duration of the poison. So you might want to only use two reagents instead of three to get a longer duration on the poison effect.
Quantity
Crafting a furnishing works the same as when you craft a potion. You need to make sure to have the right ingredients, if that is the case you can craft the furnishing. In this example here we had the Daedric Brazier, Table. Once we crafted it we can place it in our home.
Conclusion Alchemy Guide
Alchemy is an important tool in ESO to create powerful potions and poisons, but also for crafting certain furnishings. Every player in the Elder Scrolls Online will sooner or later get in touch with alchemy. You can also sell potions, poisons, furnishings or simply the ingredients in order to make gold. Certain ingredients have very high prices because they are used in lots of combinations or they are super rare.