How do I let someone submit posts?

Give them the Publisher role.

For an existing user: Users → All Users, hover the person, click Edit, change Role to Publisher, and update.

For someone new: they register on your site like any other member. Whether they land as a Publisher automatically or need promoting is a setting.

Editor is the next step up — Editors review and publish what Publishers submit. Give it only to people you’d trust to publish without a second pair of eyes.

Neither role can reach plugins, themes, users, or settings.