When you pick an affiliate, it is important that you determine which companies you will profit best from based on the frequency that the product is likely to sell. There are several types of affiliates and some will offer you more money from a sale than others will. Before you get hazy-eyed by dollar signs, however, remember some products are bound to sell more than others. If you sell on product four times a year from a little known company that gives you $100 per sale or sell a well known product 3 times a week at the rate of $10 per sale, then you will make much more money with the $10 per sale product. Often times large well known companies will offer a very small profit per sale because they are established and know that their product is likely to sell well. Small, unknown companies, on the other hand, need all of the promotion they can get. Also, because they are unknown, their products are likely to sell less frequently.
The most important rule when picking affiliates to promote on your site is to pick products that would be of interest to those who would likely visit your niche site. Otherwise you will be promoting a product to a group of people who do not even want nor need it. If your niche site targets parents of young children, then promote baby products. If you niche site targets gardeners, then promote gardening products. If you promote baby products to gardeners, you are almost guaranteed to not make a profit no matter how much money the affiliate company offers for a sale.
Lastly, there are two main ways to find an affiliate company for your niche website. You can sign up with a company that manages the accounts of hundreds of companies, big and small, and apply for the products within these accounts or you can do a web search for companies that would match well with your niche, visit their websites, and see if they advertise an affiliate program.