There are various means of heating a gas to cause ionization in order to make it a plasma. One method is simply to apply a sufficiently high potential difference, as in a neon lamp or fluorescent lightbuld, or an arc discharge.
In fusion reactors like a tokamak, one can use ohmic heating in which a large current is induced in the gas, which behaves like a wind of a transformer. Other methods include radiofrequency heating in which the electrons are heated, and their energy is then transferred by collisions with ions.
Yet another method is neutral beam heating in which gas (fuel) atoms are ionized, the ions are accelerated to several keV or 10's of keV, and then the ions are neutralized (recombined with the electrons of which they were stripped) and injected into the gas/plasma.
Ideally, if sufficient fusion reactions can be induced, then the kinetic energy of the fusion reaction would heat the plasma, which is the goal for sustainable fusion reaction.