money for reviews on amazon
Whether that be paleo cooking or DIY educational activities for teachers, the niche must be of especial interest to you - something you can constantly write about without getting tired. This gives you plenty of opportunities to naturally insert product links without being overly salesy.
Generally speaking, applying for the Amazon Affiliate program and becoming an associate isn't really the hard part. The problem is making enough sales so that you can turn your passion project into a source of income.