Necessary things for offering Arghya – Silver pot, milk, rice.
offer water like this – Fill milk in a silver pot, add rice to the milk. Stand facing the Moon God. Place a big plate on the ground and after this, while reciting the mantra Om Som Somay Namah, raise both the hands and offer Arghya to the moon from the pot. Drop a stream of milk from the pot into the plate. After offering Arghya, they can donate milk.
If there is no milk then offer water to the moon. If you do not have a silver pot, you can offer Arghya with an earthen pot.