Your best bet is to go niche. Ask yourself what specialised things you like and want to talk about? Do you think other people would want to talk about it? If yes then investigate what competition you have and go from there.
Competition isn't limited to forums either. What social media presence does your niche have? Are there Facebook groups? Discords? Are forums even the best platform for talking about your niche? Don't be afraid to look to these other technologies, they're often more convenient for people to join and contribute to.