For fusion to begin there must be some initial activating energy but, once begun, the reaction is exothermic and the energy released self perpetuates it. Fusion takes place in the sun, but for the H atoms in an aerostatic balloon to fuse to form He they must get close enough together for binding forces to appear among their nuclei (this is what they need the activating energy for). If you click here you can see a diagram of a fusion reaction.

Achieving fusion in a controlled manner presents enormous technical difficulties. A huge amount of activating energy is required, the temperature of the fuel atoms must be raised to 100 million ºC, this is why the reaction is called thermonuclear. At this temperature matter is in its plasma state, atoms in a sea of free electrons, and it cannot be held in any receptacle because none support this temperature.
Research to dominate and control the processes of fusion is still going on.
The hydrogen bomb is an example of an uncontrolled thermonuclear reaction. To initiate the reaction a conventional uranium atomic bomb is exploded to provide the necessary initial energy.