The first substance to begin the process of digestion by acting on food is

Digestion begins in the mouth and the journey of food in the digestion process takes on a fixed path, from the mouth, to the gullet, then to the stomach, the large intestine, the small intestine and finally waste materials are expelled through the anus. 
The first substance to begin the process of digestion by acting on food is the saliva.
Saliva in the mouth contains certain digestive juices. It mixes with the food with the help of the tongue. It softens the food and makes it easy to swallow.