Where can you trade Pokemon on Pokemon Diamond?

Well, for the first few games you need to have a link cable connecting you and your friend's gameboys, and talk to the lady at the desk to the right, to enter the Cable Club. In Pokemon Yellow, you can have rules or cups involved in battles. In Ds games, you talk to a lady on the upper floor, standing on the left, and she will send you to the union room. In the union room, you will see your friends, but with a different appearance (Such as Ace trainer, Idol, and so on). Talk to them, and select trade on the menu. In X and Y, you can select a passerby, and trade or battle them by selecting so. For battles in Ds games, you talk to the lady on the upper floor to the RIGHT and choose your rules and stuff, then you/your friend will become the leader and select "Create group" (Or something like that) and up to three more people (Besides the leader) can join. to join, select "Join group." HAVE FUN.