Hidden Power's power and type vary depending on the individual Pokemon using it, ranging between 30 and 70 power. There is no way to change a Pokemon's Hidden Power type or power, so you are stuck with what you have. Hidden Power is always a Special-based move.
The user throws its Held Item, removing it permanently while tossing it at the target. Power, type, and side-effect are determined by the item tossed, generally by the implied weight of the item (Iron Ball is the heaviest, and the Plates are also very heavy).