A good rule of thumb is to feed your ball python a rodent that is no bigger than the widest part of it's body. I also recommend getting two identical hides. On for the warm side and the other for the cool side.

After your ball python has settled in for a week or so with your temps and humidity being spot on offer him/her a live mouse at night. Because ball pythons are nocturnal they are most active at night.

Good luck!