To get the Pokemon you want, see if the Pokemon you want to use is available in-game. If not, you can go to the Trade Center and ask for the Pokemon you want. (I don't go to the Trade Center much, but I think there's a thread somewhere there that lets you make a quick trade or two for the Pokemon you want). And once you have the Pokemon you want, it's time to EV train them. Know your EV training spots, and use the Power items to speed up the process, if you have them. If you have Pokerus, even better.
Here's the current list of Pokemon and their respective tiers. This could change now that the move tutors are revealed, and some moves and formes could cause massive metagame changes.