I purchased a domain to build up a website for affiliate. I wonder if i should pay for business registration while i purchase domain name because i am not an organization. What should i do ?

You don't state what country or state you live in, so legally, hard to tell.

If in your shoes, I wouldn't yet. Why incur the paperwork, expense and government scrutiny in what might never be a money maker?

If your website takes off (good luck btw) then hurry and get registered.

If it is offered where you live, it might not be a bad idea to do a lookup to make sure that the name you have selected is available.

Again, good luck