The success of an affiliate marketing strategy depends on how many referrals you’re able to send to merchant sites and how well these referrals convert (hence the bolding of these factors above). The more relevant and appealing the offers you highlight on your site, the higher both your click and conversion rates will likely be. If you’re running a travel blog, you probably don’t want to be featuring affiliate offers for baby products; replacing them with affiliate links to cruise packages would probably result in a higher referral rate.
Amazon’s affiliate program is the most popular of them all. I don’t participate myself (yet) but the majority of affiliate marketers I know use Amazon because… it’s Amazon. You can review products you have used or write tutorials (eg. how to connect computer to TV) and drop an affiliate link to an HDMI cable… just a couple examples. You may want to build relationships with the manufacturers so you can get products before they’re released – giving you time to create a review before the product is launched and capture sales during peak buying times.