The two most reliable, efficient alternative energy sources are solar power and wind energy because they both come from unlimited stores and can be easily captured and turned into a useful product.
Solar power is power from the sun. The sun has been there long before humans and probably long after. We can harness the suns uv rays and capture them in PV cells (solar panels) and convert them into energy. We are doing the earth no harm by using this form of alternative energy source nor wind energy.
Wind energy is captured in wind turbines. The natural wind that flows around us can be also harnessed in a way that doesn't hurt the earth. this is done when the wind flows over the turbines propellers, which turns the shaft inside. This turns the generator which turns it into electricity. The only con with this source is that in the making it does require some of the negative energy sources in order to make it. Only after 10 years of turning will it act positively on the environment.
In conclusion the world needs to switch from fossil fuels to alternative energy sources in the next decade or we will run out of fossil fuels and if we haven't grasped the idea of alternate energy the world would be left in a sudden panic and would be powerless. The world wouldn't have the energy to make the alternative sources either so we must start now.