Which conditions are necessary for fusion to occur?

High levels of energy

an example would be the sun

For fusion, high temperature is the necessary condition.

High temperature gives hydrogen atoms enough energy to overcome the electrical repulsion between the atoms. This fusion process requires high temperature that is nearly about 100 million Kelvin and this temperature is six times hotter than the Sun's core. This high temperature provides enough energy to hydrogen atoms that they can overcome the electrical repulsion.