The first thing to do is to join an affiliate program then select the products you intend to sell. The owners of the products would provide you with an affiliate code you can use for referring traffic to the main site. Also, you will be offered banners, text links and several forms of creative copies in which you just copy and paste the code on your own site to refer traffic. Any interested customer who clicks on the links from your website will be redirected to the main site where the product is sold and if they pay for the product or subscribe to a program/service and indicate you as the referral, you get a certain percentage as commission.
Use Deep Links – these are pages on your affiliate’s website that AREN’T the homepage. For SiteGround’s hosting I link a lot to their speed technology page as an affiliate link. If you’re doing Amazon’s affiliate program you just want to gather a list of products you will be recommending to readers, create an affiliate link for each one, and import them to the plugin.
If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.