Here’s a very good guide to alchemy in The Elder Scrolls Online by Odin Online.
The video shows you how to mix various waters and reagents for their maximum effects in potions and poisons. It also goes into other benefits of leveling the alchemy skill. It’s well edited and pleasantly laid out.
I know that messing around with plants and potions doesn’t seem that cool compared to running around and hitting things, but as in past Elder Scrolls games, in TESO it can be very useful. If you do it right you can make some serious gold and help you out in a tough fight.