You're going to need Pokémon Bank, an App built for Storing and Transferring Pokémon from different games. You'll need one 3DS(Already Have) A Copy of Pokémon Black, White, Black 2, or White 2, A Copy of Pokémon X or Y and Pokémon Bank and Poke Transporter.
First, download Pokémon Bank(If you can't connect to the Internet to use the Nintendo eShop, connect to the Internet in the System Settings). There is a Yearly fee of $5.00 to use it, not a bad fee.
Then download Poké Transporter, an add on to Pokémon Bank. Start up Pokémon Bank ONCE and go through the set up with the App. once you've done that, start up Poké Transporter.
Take OUT Pokémon X, then plug in Black. Go through with the Transfer, HOWEVER you need to use the first Box in the PC like Box 1. Once Transferred, they cannot be transferred back!Your Pokémon will be in the Transfer Box on Pokémon Bank.
After that, put your Transferred Pokémon into your Copy of Pokémon X and you are set!
Hope I Helped!