Prata find out where your niche is active and be involved with them sufficiently to notice you. See if you're allowed to promote your community there, even if it requires (financial) incentive. Then make sure people stay, create a daily discussion with an interesting topic, do events like giveaways, reach out to companies that are active in the same area for long term relationships (to get products for giveaways or who could even end up sponsoring).
There's a lot of information about this topic on the net. I wrote this discussion to list some valuable resources: https://blomstra.community/d/135-feverbees-getting-started-with-community-management