Is a memoir fiction, since it's written like a story? Or non-fiction, since it's based on true events? Do I have to write the whole thing before submitting? Or do I write some samples and an outline and query, like a non-fiction book?
A memoir is non-fiction. It's often shelved in its own section, or with autobiographies, but never with fiction.
However, submission requirements for memoirs are usually the same as for fiction—meaning, you send your query and/or cover letter, synopsis and/or outline, and the first three chapters or sample pages (in order, not random chapters), as requested by the publisher's guidelines.
Some publishers have separate guidelines for memoirs, but most don't.