Users are to acquire admin approval before advertising any kind of writing contest here on the site. Also, contests cannot bypass any pre-existing site rules, such as no OC content or blacklisted subjects. The following are things to consider if you are wanting to organise a contest:
Theme
What will the core theme of your contest be? Will it be festive/seasonal, like hosting a Christmas contest in December or a summer contest in July? Or will there be some other theme, such as silence, anti-cliches, or liminal spaces?
Personnel
A minimum of two judges is necessary for a contest to take place, so assuming you yourself will be judging, who will that other person be? Do you have clear and easy offsite communication with them? Will anyone else be involved, such as narrators?
Mechanics
What kind of scoring system will your contest have? What will the judging categories be, how many will there be, and what will each category be out of, numerically? Will the contest rely on written or image-based prompts, or neither? Will there be a word limit?
Dates
Over how long a period will the contest take place? When will the scores be announced?
Prizes
How will you incentivize people's participation? Will the story get a place on the front page? Will the top-scoring stories get narrated?
And finally...
If you've considered all of the above and have posted your contest as a blog, an admin will add it to the front page to give the most exposure possible. Still, keep in mind there's a good chance that not a lot of people will come across it of their own accord. It might be a good idea to message people who you think would like to participate on their talk pages. Just remember to be polite and not spam.