For the last few years there has been an on-going debate about Pay-Per-Click (PPC) verses Search Engine Optimization (SEO). Each has its advantages.
With PPC you get immediate visitors with very little impact to your site itself. With SEO you don't have the cost incurred for paying to get visitors, but there is a significant amount of work and re-work required to make the site rise in organic searches. Plus, people tend to trust organic search more than PPC.
Google AdWords is the king of pay-per-click advertising. If you are planning to use pay-per-click as your advertising method for your affiliate products, then AdWords is probably your best choice, because Google has most of the search market. They also have the largest network of websites displaying their ads around the Internet, so your ads will get the largest exposure possible. Here are a few AdWords tips you may find useful.
Pay-per-click has a learning curve. You need to study and test. This video series provides insights into the PPC world and how to make it work for you.