Use a mail forwarding service to generate disposable e-mails used to sign up, if you accidentally give it to someone else it doesn’t expose any other accounts and can be easily replaced by a new one.
Use a password manager to ensure a strong and unique password.
Use a JavaScript blocker so you only allow the required JavaScript to make the website work and prevent automatic downloads.
I’ve been doing these in general recently and it’s good privacy a security practice. Also slowly replacing my main e-mail address in different service accounts with disposable forwarding e-mail addresses.
Edit: Other than that read a bit more about Lemmy and fediverse workings, verify what instance you’re viewing and navigate to the desired community via your instance as others have mentioned.