There are a few ways, but the first two ways have already expired.
The first two ways.
Azur flute:
Get it from a wifi event in Pokemon diamond, pearl, platinum. Use the azure flute to summon arceus at spear pillar.
Arceus:
Get it from a real life event from game stop or Toys R Us.
The third way will never expire.
AR codes. (If I find more, I will add them. But if you dont want any codes, I wont add any)
Arceus(Platinum):
94000130 FCFF0000
621BFAF0 00000000
B21BFAF0 00000000
B0000004 00000000
0000B600 EDB88320
2000B62C 00000000
Press L+R while entering the canavil pokemarket.
I don`t remember if you get the azure flute, but if you do, here are the requirements
Have all badges
Capture dialga, palkia, or giratina
beat the Pokemon league
get the national dex
Once you get it, you may trade arceus to Pokemon heartgold or soulsilver.
The fourth way
Trade someone for an arceus
