how do i go about sueing someone for defmation?

For starters, you're posting from New York (apparently). New York law, called long-arm jurisdiction, specifically excludes out-of-state defamation about someone in New York. So you'd have to sue this guy in Florida. You can write letters and make demands from New York, assuming you know who he is, but a lawsuit has be to in Florida. Being called a pedophile is probably going to qualify for libel in any state, but consult with a Florida counsel to discuss the specifics including any Florida SLAPP Act and privileges that may apply.