Develop, design, extend, document, translate Flarum or assist others in their adoption and use of Flarum.
The most direct way of helping the success of Flarum is contributing to Flarum core or the first party extensions like approval, lock or tags.
The strongest force in the Flarum ecosystem are the developers building extensions for much wanted features. The core team heavily focuses on making Flarum extensible, using that extensibility to solve the needs of the many by building and maintaining community extensions, free or paid, is a guaranteed way of being helpful to Flarum.
Documentation, extensions and language packs in your own language are a big plus for any community.
On a daily basis, people need help with Flarum and its extensions. Guidance of these users helps offload the core team so they can focus on maintenance and development.