If you're a Guest Writer, then you can access the editor by clicking the green button (+New Story).
For all other roles, the editor can be accessed from the dashboard.
Or from the stories overview page.
Then you can just start typing.
Type your title (make sure it's not too long as some channels have only limited amount of characters allowed to be used as a title).
Then start typing your text in the area under the title.
Want to know how to ad an image, video or other media files in the story, click here.