If the target uses a damaging move this turn and if the user uses Me First before the target, the user copies that move and uses it against the target, hitting with 1.5x as much power. If the target moves before the user, Me First fails.
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).