Arceus is a promotional Pokemon, so you'll have to wait for Nintendo to do an event featuring it, in your country/region. Nintendo does wi-fi events these days, so it's very easy to obtain event Pokemon when the time comes: The hard part is keeping track of the events. You can use Twitter, (follow ProjectPokemon, or something similar) or check the official Pokemon website every so often to see if an event is going on. They just had an event last month, so it might be a while before they have another one.