In summing up Xeongenesis I'm going to quote the Wikipedia entry as it sums the whole thing up beautifully.

"In 1977, 22-year-old James Cameron was a truck driver who wrote science fiction and painted small models. He saw Star Wars and was upset that someone else created a film that he felt he should have made. He studied how the effects in Star Wars were done and practiced dolly shots in his living room. He also went to the library at University of South Carolina to study special effects.

Cameron convinced several local dentists to provide an investment of $20,000, and he wrote a screenplay for a short film with a friend. He produced a 12-minute short that featured a stop-motion fight scene between a woman in an exoskeleton suit and an extraterrestrial robot, which he had built himself. Cameron hoped to present the short to studios to make a feature film, but he was unsuccessful. The effort did land Cameron a job building miniature starships for Roger Corman's film Battle Beyond The Stars and he eventually became one of Corman's visual effects specialists."

So there you have it. Here's James Cameron's first short film: Xenogenesis.