Ok, so first you want to find a good web host. I recomend yola because they offer free websites without ads. Google Sites is a ok alternative. Next, afer customizing your site, find a forum page for your site. One solution is Tal-Ki embeded fourms, these however have a major security gap and provide the names of the user unless changed. If you want a annonomus service, use SparkBB. This is a great solution, however you have to go onto a waiting list for months untill you get it, so I recomend you plan to releace this website about 1 and a 1/2 years. Trust me, you will have alot of things to do during that time. Next, you have to have a email for your gossip site. This has to be different from your regular and can't contain your name at all because if you do the site can be easily traced back to you. This is a time to bring it into view that if this site reaches the teachers or parents, you might be facing alot of troble. Now you have to find entertament for the side of your fourms. When I entertament I mean a chatroom or a MP3 Player. I would recomend Meebo, however your users will need to be 13 years of age to use it. I can't go into more detail about that, like hosting the files and customizing it for your site, but if you want you can email me for help. Once your done, publish your site. Keep in mind that your site will have a subdomain. A example of a subdomain would be "example.yolasite.com" or "example.googlesites.com". If you want a shorter domain, then use the free service dot.tk. They are a URL renaming system. You can turn example.yolasite.com to example.tk.
Tips:
Remember to limit the amount of friends on the site. If the site gets to a parent or school offical you might be in trouble or the site will be disabled from school computers.
Keep the gossip clean.
If you need help, just email me! I will be glad to help!