Well, to get shedinja, you must first level up a Nincada to level 20 with a spare slot and pokeball.
Secondly, you need to breed a female nincada/ninjask with a male pokemon that knows bug bite and silver wind in the Bug egg group. The ones that learn both of the moves by lvl up are : dustox, beautifly, volcarona.
PS. dont use silverwind on shedinja, it's special atk sucks