All diaries are automatically posted.
Stories, which appear on the front page of the site, are different. They are intended to be of higher quality than the diaries, since they last longer and are the first thing anyone sees. Therefore they are subject to voting. Users get to vote whether a story appears or not.
If a story is voted up, it appears on the front page. If it is voted down, it disappears completely.
Diaries can be posted to the front page also, and will appear there along with other stories. This is so that high quality diaries can be shared more widely. You don't need to do anything special to activate this feature, it is automatic. The "Vote Diary to Front Page?" just needs a certain majority of users to vote yes. If the diary is voted to the front page, it will also still feature in the Diaries section. If the diary isn't voted to the front page, it will just stay in the diary section, unless it is dumped to The Hole by an Editor.
It is considered bad form to post a story if you have not previously participated in the site. HuSi is mostly about the diaries and unless the story is truly outstanding, a story submitted by a user with little prior history will be voted down quickly. If you are at all unsure whether or not something is appropriate as a story, post a diary asking.