Affiliate Marketing Secrets

I run various e-commerce websites and I’d like to branch out and have a referral (affiliate) program. I’m from the United States and my affiliates would be from all around the world. I was wondering from a legal and tax standpoint, what information I would be required to gather from my users. Would it be different for those in the USA, opposed to those outside the USA? Would I be required to collect SSN #’s or would there be a way around that?
I tried to do some research on this, but can’t seem to find any information on the matter. Hopefully someone can enlighten me.
Thanks in advance!