I hear this question from affiliate beginners all the time. My question to you is this: “What do you want to promote?” There are affiliate programs for just about everything under the sun, so the decision is up to you. I suggest starting with an area you are familiar with. Simply do a search engine query for the product you are interested in, plus the words “affiliate programs”. Example: “Stuffed Animals Affiliate Programs”.
You can also find products on the home page of AffiliateWatch. We curate some of the top referral programs out there.
You can use affiliate networks. Affiliate networks list many programs similar to Affiliate Watch does. I know they even show a few of the partners on this website. I like clickbank and CJ.