When hydrogen reacts with oxygen to make water, it is an exothermic reaction, meaning it produces energy. This is exactly what we want for our cars. A problem arises however when we want to turn the water back into hydrogen and oxygen. This can easily be done using a method known as electrolysis, but since electrolysis is an endothermic reaction, it uses up energy. Unfortunatly, a lot of energy is required to produce a substantial amount of hydrogen fuel. We don't have a lot of energy to spare at this time, so we must find alternative ways of producing energy. The most popular idea now is solar power, and there is lots of research being done on using the energy of the sun to split up water molecules.
