You can start writing about your free classified website on various online platform such as
Questions and Answer websites
Forums
You start sharing your site on social media websites such as
facebook
twitter
tumblr
Browse various classified websites and collect the email address and phone number of the ad poster. Try them through phone call, sms and email and tell them your the features of your free classified websites and convinced them to post ads on your websites.
Try to make your free classified website simple and user friendly so people cant face any difficulty while browsing your site.
The main user of a free classified websites are ad poster so try to make ad posting page very good and they able to post their ads without facing any problem.
The complicated ad posting process irritates the ad poster and they start avoiding these types of sites.