A snake's food chain begins with sunlight, which helps wheat and grain grow. A small animal such as a mouse then eats the grain, a snake eats the mouse, and a larger predator such as an owl eats the snake.
When the snake dies, its body decomposes and becomes rich nutrients needed for plants to grow. Decomposers, such as worms, are the end of the food chain. The process repeats itself in perpetuity. This is the general process of every type of food chain.
All food chains must start with a producer, and all other organisms in a food chain are consumers. Food webs are more complicated, involving many different food chains.