Saliva in the mouth contains an enzyme called amylase that catalyzes the breakdown of dietary starch into simple sugars. This is the first chemical step in digestion. Amylase is made by the pancreas and secreted by the salivary glands.
Starch is a polymer of simple sugars. A polymer is a very long molecule with repeating units, in this case monosaccharides or simple sugars. Amylase breaks the starch molecules into disaccharides and trisaccharides, molecules made of two and three simple sugars respectively. These molecules are further broken down into monosaccharides, mostly glucose, by other enzymes.
You may have noticed starchy foods starting to taste slightly sweet as you chew them. This is because their starch is being converted to sugar in your mouth.