It's not "creating" energy, it's releasing energy.
The original atoms (exactly which atoms depends on the reactor, but let's assume it's deuterium and tritium) have a certain starting energy. When you fuse them together the resulting atom (helium-4, if you start with deuterium and tritium) moves it into a lower energy state.
Since the fused atom has lower energy than the input atoms, the fusion reaction releases the difference in energy, which you can then capture.
The original atoms (exactly which atoms depends on the reactor, but let's assume it's deuterium and tritium) have a certain starting energy. When you fuse them together the resulting atom (helium-4, if you start with deuterium and tritium) moves it into a lower energy state.
Since the fused atom has lower energy than the input atoms, the fusion reaction releases the difference in energy, which you can then capture.