What starts protection?
Copyright protection generally begins when a human author creates original expression and fixes it in a sufficiently permanent form. A poem in a saved document can be protected before it is published. An idea for a poem, by itself, is not protected as copyrightable expression.
Does publication change ownership?
Publishing a work does not, by itself, mean that you have assigned its copyright to the publisher. However, a contract can grant or transfer rights. Employee-created works and some commissioned works may qualify as “works made for hire,” with a different starting ownership rule.
What about registration?
Copyright exists without registration, but registration can provide important enforcement benefits. In general, registration (or refusal of registration) is needed before filing a U.S. copyright infringement lawsuit concerning a U.S. work. Timely registration can also affect the remedies available.
What this means for a journal contributor
Keep the final submitted file, publication date, acceptance message, and the journal's agreement. Those records help distinguish what you created from what you agreed a publisher could do with it.