Depends on what you're looking for, but there's two basic options - Adaptavist's Community Forums and Atlassian's Questions addon.
They are different, forums really is aimed at replicating the multi-user, multi-thread structure most other forum software implements, and Questions is very much more like a support/Q&A site. I'd try looking at both though.
(Disclaimer here - I work for Adaptavist. Please don't take this as a sales pitch, I'm just pointing out that it's intended to do forums in Confluence and might suit you. I don't actually work with it much myself, I'm in a different area, although I have worked with older versions and it worked well then)
Hi team, I’m Avinoam, a product manager on Confluence Cloud, and today I’m really excited to let the Community know that all customers can now try out the new editing experience and see some of the ...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs