Hacks in the forums are distributed in patches, usually in IPS format. You can check out the FAQ thread, or google how to create or apply IPS patches. You can attach this patch to your post when you make a proper thread for your hack.
Now, there are different sections here where you can post your hack but it depends on what kind of hack it is and how far you are into it. If you have more than 4 gyms in your available release you can try posting it in Hacks Showcase, otherwise just post it in Progressing Hacks. If you're still very early in its development and you simply want some feedback on your ideas, you can post in the Beginner's Lounge. Side projects like 386/catch them all hacks go to the Sideshow Showcase. Keep in mind that threads in these sections, with the exception of Beginner's Lounge, are under moderation, which means a moderator would have to approve your thread before everyone can view it. Your thread will have to follow some requirements, you can view these in the Rules thread of each section.
I'm going to close this thread now since this section is mostly for general ROM Hacking discussions. If you have any more questions regarding posting a thread for your hack, feel free to message me or any of the ROM Hacking moderators. For any ROM Hacking help in general, you can make a thread in Beginner's Lounge. Good luck with your hack. ^_^