They need to have sufficient thermal energy to overcome their like-charge repulsion force and be contained for a certain amount of time to allow a fusion collision to take place. As we all have observed, gases like to fill the volume of their container. Plasmas are no exception, but the incredible temperatures of fusion plasmas make it impossible to confine with any material wall. Scientists thought about how to make a container that could possibly hold a 100,000,000 C substance. They look at the sun and other stars for ideas.
Stars use gravity to balance the kinetic pressure or expanding force of the plasma.
The gravity is supplied by the mass of the star itself. Here on earth we don't have the option of using gravity, but since a plasma is a collection of charged particles electromagnetic forces may be used instead. It is possible to create a magnetic container to hold the plasma, this forms an equilibrium between the kinetic pressure and the magnetic pressure.