Question

What are the starting materials for nuclear fission

Answer 1

A neutron and a large nucleus.

Answer 2

In normal fission reactors, the fuel used to start the nuclear fission is Uranium-235. 
Generally, fuel rods enriched with uranium-235 are used to start the fission. When a nucleus of uranium-235 absorbs a neutron, it becomes unstable and then it breaks apart, producing two smaller nuclei, several neutrons and energy. The additional neutrons produced in the reaction are then absorbed by other nuclei of uranium-235, triggering other fission reactions, and so on.