You get payed to bring customers to a business. If you are talking about doing it over the internet then you need a website/blog/podcast/video podcast/etc with a good number of visitors. You then try to drive those customers to a business through ads and other endorsements.
You'll need to know about webdesign, search engine optimization, writing persuasively, finding cheap and reliable hosting, and you'll need engaging content that will attract the right demographic and stand out in the vast sea of content that is the internet.
The list of seedbanks on this site would be a good example of how it could be done. Disclaimer: I do not know if they have an affiliate deal with any of the seed-banks listed. I'm just saying it would be a good way to do it as an example of how it can be done.
I'll give a real world example. Eve online is an MMORPG that hires affiliate marketers. They give $10 for every person that clicks your link and buys a copy of the game with their account within 30 days. (I'm not endorsing this deal I am just using it as an example)
https://www.eveonline.com/affiliate/default.aspx
Alternatively you could just try spamming but that's just a separate subset of technical skills. Also it's bad so don't do it.
