A product review site needs to provide value to its visitors, and one of the best ways to do this is by creating how-to guides. Select a topic around which you want to create an affiliate website. Create useful tutorials for your readers, keeping your affiliate product as a problem solver.
With every order confirmation or delivery email you sent to your customer, you can tell them about your affiliate network. There is a pretty good chance that if your customer is happy with your product or service, he is more than willing to promote your product.

Choosing an affiliate program is an incredibly important step. This can also be a little bit misleading as very few top notch websites only choose one affiliate program. Many big websites focusing on physical products are going to be Amazon Associates. Depending on the niche they might also look at other affiliate programs or networks that are specialized.
Again, the most important secret to success as an affiliate marketer is creating value. Don't approach it as a money-making business. Instead, think of yourself as a helpful encyclopedia for your audience.