Online affiliate marketing have reached an (my guess) estimation $3.0 billion (or more) of payouts between 2016-2017 to webmasters/"blog"masters.
From a humble beginning in the late 90s, it has now become one of the main source of "passive" income that can be done on a part-time basis - even on full time.
Basically it requires you to have :
a. A blog/website - either paid or free with ads - to me, that's not really important - what is more important is how to draw crowd to your site. (Keywords, Hashtags, Labels, Meta etc)
b. Products/Services - Be specific and put down some information on what you really do or what you are really good at. It helps to state your experience and successful stories as well. Additionally, photos, tables and diagrams will enhance your internet presence.
c. Affiliate with those selling similar Products/Services
Note : It helps to first google "e-book affiliate" - and understand what it is all about
This is the part where you need to choose which affiliate that suits you most...(do not be greedy..I've seen one simple banner on someone's website that have made so much money).
There are so many of them but I like to recommend you :
a) PayDotCom
b) Google AdSense
c) Commission Junction
d) E-Junkie
and many more.
(Note : You should have PayPal account by now!)
Affiliate yourself with merchants with site publishers and help them sell their products/services (Tip : Find something similar to your products/services - because you know better what you're good at)
Vice-versa - ask merchants to help sell your product as well.
There are many more things to learn about online affiliate marketing...there are so many resources for you to read and understand.