In practice only a fraction of the energy locked in matter can be tapped by nuclear reactions. Only by combining equal amounts of matter and antimatter can complete conversion occur. Nuclear fission releases a tiny fraction of the available energy by breaking heavy nuclei like Uranium apart. This process is the basis of nuclear reactors and power plants. At the moment, science has not found a way to control fusion reactions for power generation.
Fusion is difficult to control because the reactions happen at temperatures of millions of degrees.
A Helium nucleus consists of 2 protons and 2 neutrons. Two of the protons will be converted to neutrons in the process. Two protons thrown gently at each other will bounce off. If they get close enough, however, they will they will stick They must be thrown together at extremely high speeds. High temperatures imply high speeds for gas particles.