How do you i get arceus

1.Get the azure flute from a Pokemon event. 2. Go to any pokemart to get the azure flute from the postman. 3. Go to spear pillar (if you have diamond, pearl or platinum then follow the instructions given to get Arceus, if it is from a different game, I have no idea). 4. Walk up and it will ask if you want to use the azure flute, press yes. 5. Walk up the stairs until you reach Arceus. 6. Talk to Arceus to fight it. 7. Just try to catch it (remember to stock up on ultra balls).

Simpler method: Trade (trade with your most precious Pokemon, I would suggest trading with a cheater that has lots of Arceuses).