You'll need to register with the US copyright office as soon as possible. Not doing so means that any jerk sending a DMCA, legitimate or otherwise, can sue you personally and win, even if you prove in court that you had no knowledge of the specific instance they are suing over. This comes up more than once a year on English-language social media sites with as few as 1000 users, which can be shocking to people new to operating social media and/or forums.

There are also other relevant laws like COPPA, FOSTA-SESTA and GDPR, but the timeframe for those is not particularly urgent. Additionally, if you aren't retaining legal counsel and are handling things yourself, the Australian Communications Alliance has some handy explainers on compliance with these other laws for site owners.