Lemmy is by default set to “local”, so a lot of users will see posts from a new community, especially if it becomes popular. As for that the key is to simple, short, visual content so basically memes and the like. The other content going well are important news. So if you post those in your community, you might get a lot of interaction. If you want the community to somewhat grow, you probably want to post a lot on it on a regular bases. Then crossposts are important as well. That can bring people to your community.