Fusion takes place in the core of the Sun where the required temperature and density are produced by the gravitational forces of the Sun's great mass.
Hydrogen nuclei combine into helium nuclei when the repulsive force of positively charged protons is overcome allowing the strong force to take hold and bind them together. In fusion of hydrogen to helium, half of the protons are converted to neutrons and the �extra� energy is released.
Four hydrogen nuclei (one proton each) combine to �build� one helium nuclei (two protons and two neutrons).
Sustained fusion that produces more energy than that required to keep it going has not been achieved at room temperature. Development of economical energy production from fusion (at any temperature) is still (after 50 years of research) very problematic.
http://en.wikipedia.org/wiki/Fusion_power