[color=magenta]Wow, it has been a long time since you have been here at TPM. Welcome back!!
To get a Shedinja, you need to level up a Nincada to Level 20, which is when it evolves into Ninjask. At this time, you will also need at least one open spot in your party, or else you will not get a Shedinja. (However, the widespread rumor that you need at least one PokéBall in your bag to obtain Shedinja is not true.) When your Nincada evolves into Ninjask, you will also have a Shedinja in your party, providing that you left an open space for it. If you let your Nincada evolve without having an open space in your party, you will miss out on getting a Shedinja, and you will have to raise another Nincada.