We will assume that you have selected create content and chosen "story" as a content type.
You should be looking at a form with the title "Submit story". From here, it is just a matter of filling in the form and posting it.
That wasn't too painful, was it?
Decide where you want the "teaser" (the part of the main text used in links to the article) to end. If you do nothing, Drupal will choose a breaking point for you--but it's better to decide yourself, to make sure the breaking point is appropriate. You do this by clicking the Break button that looks like this:
where the teaser is to end. If TinyMCE is not working for you, entering <!--break--> does the same thing.
And you're set! You can preview the page you've prepared by hitting "Preview" (recommended, and sometimes required) or you can bravely or recklessly just go ahead and publish it by hitting "Submit".
After you hit "Submit" your content will go into a "queue" for approval. This prevents inappropriate content from being posted on the Website.