There are a huge number of questions in beginner support subs like this one, and Lemmy and lemmy-support on lemmy.ml. An easy thing to do is to subscribe to those subs and start learning and answering repeat questions both about how to get Lemmy humming on your first day (posting links to federation and community finding explainers) and expectation setting (everyone is overloaded/broken today, and helping people set realistic expectations around lack of polish compared to reddit).

These are good first tasks, because they don't require a busy mod/admin to give you access or tell you want to do... but a lot of those questions are things they'll try to get to answering if no one else does, and if they're covered then admins/mods can focus on more important stuff only they have access/knowledge to handle.