How do you catch piplup?

If you're talking about Pokemon Pearl, in the beginning of the game, there is a piplup, chimchar, and turtwig. If you choose the piplup, you'll have to wait for it to evolve into a prinplup, and then an empoleon. There you have it. OR you can trade something for an empoleon with your friend that has a Pokemon game in the diamond, pearl, and platinum series.