Its simple yet complicated. now if the person you want to play online with has the same umd both of u should turn on ur wlan and select multiplayer in game selection menu or if the person u want to play online with doesn't have the same umd but the umd you have has a gaame sharing option u guys should turn on ur wlan and the person with the umd should select gamesharing in the game selection menu then the other should go to gamesharing in the home screen menu.
hope this works for u