From the backend of your Joomla site select Components -> Advertisement board -> Category. Press button "New".Fill out all required fields and press "Save & Close".
Please, note that only if you have Advertisement board PRO, you can allow access to category only for certain groups of users.
Go to the tab "Ads" and press button "New". Fill out all required field. Note that you can attach files, images and link in your ad.
Press button "Save & Close".