There are lots of different affiliate programs out there & they vary both in the merchandise they offer and in the quality of the program. Here are a couple tips for choosing the right affiliate program for you
Does the company offer products related to your website?
Some affiliate programs enable you to sell products from multiple companies - which companies and products are available?
Does the company make it easy to include links to their products, or do you need an in depth understanding of HTML?
What % commission do you get on purchases
Does the affiliate have consumer friendly policies - will your customers feel comfortable buying from them?
Can you create a button that ads the item directly to the customer's cart right from your website?