Contact a Workgroup. Almost everything in Monero is managed by workgroups, which are groups of contributors (often lead by a coordinator) working on some specific aspect of the development. Some examples are: the localization workgroup (translations), the community workgroup, the GUI workgroup, the Outreach workgroup and so on. Workgroups are mostly independent and have their own structure. Contact the workgroup that interests you and ask how you can help. For a list of contacts see the
Community of Monero developers. Contains libraries and resources and guides of some Monero Workgroups, like the Localization Workgroup and the Outreach Workgroup.