There are lots of different ways to save energy and lots of different reasons why we should.
The first and probably the most influential is that WE save money! As a secondary, though no less important, reason is that many of the resources that we use to create energy are NON RENEWABLE - e.g. coal,oil and gas. These resources are not infinite and will run out, some people think that oil could run out within the next 40 - 60 yrs. if we continue to use it at the same rate.
Another important factor for us to consider and to encourage us to save energy is that we reduce the creation of pollution, the destruction of habitats and the endangerment of fauna; as we reduce the need for more mining, the need for the destruction of habitats to facilitate the extraction of these resources or the transportation of the resources.