There's quite a few imageboards out there though most of them are pretty dead like late.city and comfy.chat. They're interconnected so once you find one there's a dozen more under /friends/ etc.
As a teen I used forumieren.com to find and make forums. There's probably still forums floating around if you want something you can have a profile on. Try to find something for an interest/hobby you have.
Sidenote, has anyone here ever been on Livewire? I used it around the tailend of the site, 2012 or 2013 but it was a lot of fun.