In order to use 'extraction' actions you will need to locate a 'raw material source'. Sources have a green phosphorescent glow and a green name over them. Sources appear randomly in some areas of the world. Their appearance can also be provoked by players with prospecting actions.
You have a prospecting action only if you started with enough 'forage' action packs. If you don't have the prospecting action, at first you will have to extract either some randomly-appearing sources or extract sources prospected by another homin. After some experience in extracting, your Forager Trainer will accept to teach you a prospecting action.
However, if you didn't choose any 'forage' action pack and you want to retrain into a forager career, you will have to learn even an extracting action from your favorite trainer.
When your skill will be sufficient, your Forager Trainer will teach you new prospecting actions. Gradually, they will allow you to refine your search...
Once you can find or provoke the appearance of a raw material source, see How to extract raw materials.