how to make an affiliate button so when users buy something the person that?

[Bob Saget]

is helping promote my item get paid by paypal automatically?

[Jake]

You will probably need to add some affiliate program management software to your site or use an affiliate management servie, paying immediately is a rare way to arrange things, there's a higher chance of fraud going undetected compared to paying monthly after a review of the refund rates, etc. for that affiliate.

The first couple Google results:

Some 2 year old video http://www.youtube.com/watch?v=bJydoXAKZ80

http://www.osiaffiliate.com/ $27 / mo service

Some affiliate management service I never herd of: http://www.hasoffers.com/

More digging should turn up some really cheap scripts...