The basic auxiliary equipment are potions. They are divided into basic (they improve regeneration, mana amount), tonics (resistances and constitution) and grenades. Each of the characters has his own set of two potions, where the regeneration potion is the only to be free and it replenishes after each stay at the camp. The number of potions and an additional potion slot are available to unlock with perk points and Inquisition points. To brew a potion, you only need to find a brewing table (at the herbalists in the HQ, at strongholds and in outposts) pick the potion (drag it over to the appropriate field) and click it. Above, you will see the amount of the material that you need to brew the potion. To be able to brew potions, you will need plants. Their occurrence depends on the region and on the nobility. Still, it is worthwhile to pick every plant, because each may be useful in the future.
Each potion can be upgraded, but it requires many plants of a given kind. To upgrade, you only need to pick the potion and see the details to learn about the list of the available upgrades. To speed up the process of gathering ingredients, you can use the Optimal Cutting Inquisition perk (Secrets category), which gives you a 10% chance of picking more plants at one cut. Also, you can enter the tactical mode and have the party pick the ingredients for you (this takes around 11 minutes).
Another interesting option is to grow plants (it requires the Skyhold). In the garden, there are two plant pots - you can plant there any plants that grow from below the ground (as opposed to the ones that hang from above) - both the common and the unique ones. Of course, it is best to plant rare species. In the Inquisition tasks, you will also receive one that relates to the option to upgrade your garden. After you complete it, you can expand your garden - either receive more pots, to plant more plants, or turn the garden into one accessible to the citizens, who will offer the Inquisition gold and ingredients in return.