You don't need any energy at all to get fission to occur -- the fissionable substance is already at the top of its activation energy curve and is thus unstable. You can imagine that a fissile nucleus already has internal pieces which are repelling each other naturally, and it just takes a trigger, like a neutron, to allow it to break apart.
Fusion fuels, like hydrogen, are already quite stable. You have to push them up and over a very large activation energy before they will fuse, eventually releasing more energy than you put in. The hydrogen nuclei repel each other very strongly, so you have to use very large temperatures to make them move very fast. When they move fast enough, their kinetic energy overcomes their repulsion and allows them to fuse.
Hi! Thanks for your explanation! Well, i also wonder, why do we have more energy after a fusion reaction than a fission reaction? I might be wrong, i know, there is no fusion reactor because of the strong energy it spreads.