Step 1: Obtain your friend's Friend Codes and put them into your Pal Pad
Step 2: Give your Friend Code to your friends so they can put it in their Pal Pads.
Step 3: You and your friends go to the basement of a Pokemon Center and connect to wi-fi.
Step 4: You should your friends that are connected.
Step 5: You can choose to set up a battle/trade or accept another friend's invitation to battle/trade.
To know your own Friend Code, you must go upstairs in a Pokémon Center. One of the women will stop you and ask you if you want to set up your wireless, IIRC.