Trading is a pivitol part in Pokémon to get you to complete your Pokédex...in the game there are several trade areas where you are capable of trading for in-game Pokémon. They always come attached with a bit of Mail and are always have the same EVs and levels as the Pokémon you sent over. So if you sent a Lv. 5 Skitty over, you'ld get a Lv. 5 Meowth:

This trainer in a house on Fortree City is looking for a Volbeat

Pokémon You Send		Pokémon They Send
	#313 Volbeat	#311 Plusle Wood Mail Attached

This trainer in a house on Pacifidlog is looking for a Bagon

Pokémon You Send		Pokémon They Send
	#371 Bagon	#116 Horsea Wave Mail Attached

This trainer in a house in the Battle Frontier is looking for a Skitty

Pokémon You Send		Pokémon They Send
	#300 Skitty	#052 Meowth Retro Mail Attached