Legal Question in Intellectual Property in California

If your story is based on the original movie, it would probably be considered a derivative work and you need permission to do that. And, the character ET (as well as some of the other characters and images in the film) are trademarks that you would not be free to use in the resulting motion picture without proper permission.

That said, any new material you add may be copyrighted in your name and you would fill out the proper copyright form and register it. Read the instructions for the form carefully to make sure you refer to your contribution correctly. More information is available on the copyright website.

You must realize that this will be a tough sell, even if you copyright your work. No producer is going to risk a lawsuit from someone with deep pockets.

Apart from the legalities, keep in mind that ET came out nearly 30 years ago--there is a reason that no sequel has been done, and it may have nothing to do with rights.