Welcome and def read through that link Brian provided.
And assume you live in California and plans to register the truck in California, the TL;DR is to buy a NAS (North America Spec) truck. That's the most painless way to own and register a defender in California.
And give up dreams of a diesel Defender in this state.
There are alternate ways to own an imported truck, but you should be ready to make lifestyle changes to accommodate. Good luck!