Two critical requirements must be met before light elements, like hydrogen or helium, will fuse and form heavier elements. the two requirements for nuclear fusion are

In order to achieve a nuclear fusion, one of the following requirements that must be attained are high temperature and high pressure.

High temperature - temperature must be 100 million degrees kelvin to a point that hydrogen already becomes plasma

High pressure - extremely high pressure is required so that the distance between hydrogen atoms must be at the required 1 picometer apart