Instructions
Visit the Power Plant and talk to the worker by the door. He will tell you about a thief who stole an important Machine Part from the Power Plant.
Go to the Cerulean City Gym and head to the uppermost left side of the room. Face the piles of inner tubes and press the "A" button to find the Machine Part that the Team Rocket thief hid there.
Return to the Power Plant and give the Machine Part to the man on the ledge. The Power Plant will turn on and you will receive a TM 57 to teach a Pokemon the move "Charge Beam."