How can I copyright my story when I haven't written it yet?

So I finished writing the outline for the Revolutionary War story. Right now it’s only in outline form and a few character sketches. I keep my outline in a Google Doc document, and I shared it with a few jelly friends and one real life friend who didn’t seem to have a high opinion about it. I just talked to a jelly friend and she was concerned that I shared it with my real life friend, because she is afraid someone could get a hold of my link and steal my work. While I don’t think my friend would ever let my story be stolen, the conversation does raise an interesting issue for me. Could someone ever get a hold of the outline and steal my work?

My jelly friend suggested that I should ask a question on here because there may be people with knowledge about copyright. So what should I do in my situation?

FYI, I have no plan to public my story locally. I have seen a few online publishers that accept international authors and I’m aiming for that route.