To get Arceus, you have to do a few things:
1: Get all 8 Badges
2: Beat The Elite 4
3: Get The National Upgrade from Professor Oak (National Dex)
4: Get Dialga,Palkia,Giratina,Heatran,Darkrai,Mesprit,Uxie,Azelf,Moltres, Articuno,Zapdos,Cresselia,and Shaymin
5: Go to Veilstone Department
6: Go to Top Floor
7: Talk to the Old Man
8: Go to Celestic Town
9: Talk to Cynthia (She will give you the Azure Flute)
10: Go to the Top of Mt. Cornet
11: Use the Azure Flut on the little square with the - (minus) sign
12: Walk up the stairs ( May take a while, I reccomend using your bike in the fast gear)
13: Save before you battle Arceus
14: Use your Master Ball (If you still have it)
(15-17 for people that don't have Master Ball): Get your Starter pokemon (Infernape,Empoleon,Torterra) To Learn Endevor
15: Get the pokemon that knows Endevor to 1 hp
16: Use Endevor on Arceus
17: After Arceus Attacks, (your pkmn will be ko'd) use a ball (Pokeball, not a good idea,Great Ball, o.k.,or Ultra Ball, the highest thing next to a Master Ball) and try to catch Arceus ( might take a while, just hang in there)
18: If you still have your Master Ball, (It's a highly good idea to do this)
USE IT!!!!!!!
And there you go, how to get Arceus. I don't want those "10 points" or what-ever
you are welcome Matt Rocchio