The most likely thing to happen is the end of life as we know it, rather than the end of the world. As the world is now, before too long the amount of people living on it is going to be greater than the amount of people the earth can support. That is, there won’t be enough food and energy so that everybody can survive.
This has happened in the past, in smaller ways. Back in history, we couldn’t get as much energy and food from the land, and so communities have experienced this lack of energy food for their people. Many people believe that this is the cause of major disease outbreaks – such as the plague. So this is something that could happen again in the future.
Otherwise, the most likely way of the end of the world will probably human destruction – i.e. a bomb or similar