One person, home, business at a time would be a great start! We installed 12 Solar Panels with Inverter system on our roof just 2 months ago and our electric bill has already gone down 70%!:-). Also have Solar Hot Water system, works great! Considering we live in THE highest cost per kwh in all US states and possessions, its a very large payback!:-) Waiting for Governments to come to feasible/ workable agreement on alternative energy solutions AND implementation would take forever, heck they cant agree on anything! If however everyone begins by taking steps as we did, it would add up to quite a large reduction in fossil fuels. The only real worldwide solution would be to get over our fears of Nuclear Energy and start building the Newer much Safer and efficient Nuclear Power plants!
Wind, solar and hydropower are used to produce electricity. That electricity is used to power not only homes but cars. Now we just need to figure a way to do it so a car can run all day allowing people to travel on road trips.
The alternative energy can only replace fossil fuels if there was a developed capability to efficiently and economically capture and use renewable energy. It will also be a bit expensive to replace the fossil fuels.