Hobbies And Interests
Nintendo Event
Arceus is primarily a pokemon released as part of special events. There have only been six events in which Arceus was given away to players, and of those only one has been in North America. With the release of the "White" and "Black" versions of the pokemon games, further events to release an Arceus are unlikely. However, older legendary pokemon continue to be occasionally given away at events, so this is not a certainty.
Trade
You may be able to get your hands on an Arceus is to trade with a friend for one. Considering the rarity of Arceus, you'll either need a friend who happens to have two or more likely a friend who no longer cares to keep one. Another possibility is that your friend may only be willing to complete a touch-pass trade, a form of trading where the pokemon is traded from one player to another and then back to give both players the pokemon's Pokedex entry. This type of trade is best for letting players search for a specific pokemon on the GTS system.
GTS
The Global Trade System (GTS) is an online trading system available to pokemon players. Players can search for a particular pokemon being offered by other players and trade for that pokemon or post their own listings with a desired trade. The difficult part of this system is that you can only search for pokemon already listed in your Pokedex, making certain legendary pokemon like Arceus impossible to locate without first completing the aforementioned touch-pass trade. However, after you have the listing and provided you have a trader's desired Pokemon, you'll be able to obtain your own Arceus.
Pokemon Generator
If no other options exist and you care more about having a pokemon that looks like an Arceus as opposed to a legitimate Arceus, you can use a cheat system. The Pokemon Generator software takes advantage of the GTS' DNS server and lets users craft their own Pokemon from scratch and send it as though it were a legitimate GTS trade. Be aware that Pokemon obtained this way can't be traded via GTS and are flagged for tournament play as illegal.
Action Replay
Another cheat system option is Action Replay (AR). Using the AR system and the appropriate code, a player can send an Arceus to themselves to pick up. AR Arceus are also flagged as illegal and are impossible to trade, and feature less customization options as opposed to Pokemon made with Pokemon Generator.