Simple...
Fusion: find a way to get two atomic nuclei close enough to each other that the strong force pulls them together, creating a new, single particle. The most common approach is to increase the particle's RMS speed (IE, REALLY HOT) and hope that they collide with enough velocity to overcome the electromagnetic and weak forces. We can already do this, vut we get less energy out from the fusion reaction than we put in.
Cold Fusion: Same as above, but do it without using heat.