All energy used by living organisms on Earth is ultimately derived from the Sun. So you could say that solar energy in the form of electromagnetic radiation is the original source of energy for living organisms on the planet. Producers, or plants capable of making their own food through photosynthesis or chemosynthesis, are then capable of taking the energy from the Sun and converting it into food, a form of chemical energy. The energy in the food is then passed from trophic level to trophic level along the food chain until reached by the end consumer (like humans). Upon death, the chemical energy remaining in an organism is transferred to decomposers, or the bacteria and fungi that feed on dead creatures for nutrients. None of these processes is particularly efficient, so plenty of energy is wasted at each step. Less that one percent of solar energy that reached the planet's surface is ultimately converted by plants to food. And only a fraction of the chemical energy in food is passed from one consumer to another, thus food chains have a very finite length. Energy that is lost in these processes is transferred into heat (thermal energy) and sound. Most of the energy loss is in the form of heat which is simply dissipated from the organisms into the atmosphere and ultimately the universe. And the energy that organisms obtain from food is used to fuel various chemical reactions in their bodies and mechanical energy in the form of motions that their bodies perform.