1. You need to get at least 1 point out of 4 of the Herb skills. 2. When that is done, there should a new square down at the quickbar that has been filled out by a green leaf, if that didn't happen then open the Skills window and drag down the icon to the quickbar. 3. Press the Herb icon at the quickbar to open up the potions menu and when you press one of the recipes (that are located at the left side of the window that opens up), the right side should list the recipe's required to make that chosen potion. (However, if some or all of the recipes are written transparently, it means that you do not meet some of the requirements, change this by clicking at the recipes where you lack the necessary requirements in order to know what specific recipe you lack)
NOTICE: Vendors sell recipes so do keep an eye out for them, although they can be expensive. Master Varathorn at the Dalish Camp in the Brecilian Forest: sells unlimited toxin extracts, deathroot extracts and most importantly the elroot. Quartermaster at the Circle Tower: Sells unlimited Lyrium Dust should you want to create lyrium potions. Bodahn At party camp: Sells unlimited Corrupter Agent, Concentrator Agent and Flasks.