Organisms obtain all of their energy from their environment. But not all organisms obtain energy the same way. All organisms ultimately depent on the sun for the energy to sustain them. As the sun shines, about one percent of the light that hits the Earth is converted through photosynthesis into plant matter. Other non-photosynthetic organisms (herbivores) then obtain their energy by eating the plants and using the plant matter to fuel the reactions they need to survive. Then carnivores will eat the herbivores for their food. Because of this flow of energy, ultimately depend on the sun for life either directly or indirectly.
The processes of how different organisms use this energy depends on the organism. Plants will directly convert radiant energy (sunlight) into chemical energy (glucose). This process is called photosynthesis. Other organisms take plant matter produced from photosynthesis and use that matter to fuel their own biological processes. These types of organisms have to use a different process called cellular respiration to get their energy. Regardless, these processes do not occur in a simple, single step process. These processes take many steps to get their job accomplished. The processes involve the production of a chemical that can be used by the cell. That chemical is ATP (Adenosine Triphosphate.)