In the beginning of Back to the Future, Doc Brown apparently steals plutonium (from a government facility?) to power his time machine.
How does he steal it? Plutonium is a highly sensitive material and it's generally kept under heavy protection.
He didn't steal it. The Libyan terrorists stole it; he merely agreed to make them a nuclear weapon with it (which he obviously didn't!).
How the Libyan terrorists stole it is not mentioned in the film.