Beat all 56 gyms then go to lakesville city and go as far away from the jester as you can. Then wait and you will find. HOPE!!
After you get all the badges, go to that city place (the lake city thing) and just walk around, you'll eventually find an arceus.
You go to canalave city then go into the poke-mart. There will be a guy by the desk. Talk to him and he will give you the azure flute. Then go to spear pillar and it will say do you want to use the azure flute press a and it will make a sound. Then there is a staircase that leads to ARCEUS! There will be an exclamation mark, then walk forward and press a. Then you battle ARCEUS and battle until it is at low damage, then use the great ball and then ARCEUS will be captured.
Look for him after you get the bages...happy?
Well you you go on the secret cave you when randommm pokemon will comethen you find arceus
You can't get it at a lake