Actually, there are thousands of affiliate programs you can choose from, if you already have a website, search for an affiliate program related to your website niche. For example: if you have a website talking about weight loss, just go to Google and type "Weight loss affiliate" and you will find hundreds even thousands of good affiliate programs you can join for free. Every affiliate program will give you a special link that you can promote the product through it in your website.
I'd recommend you to choose an affiliate program that pays you a good commission like $20 to $100 per the single sell. You should pick only affiliate programs that pay such amount of money as you'll work hard and this is the smallest amount of money that will satisfy you. Try also to stick with affiliate programs that take care of their affiliates, such as giving their affiliate some valuable tools like free tips, free articles, online support, ready websites, helpful forums, etc.
These affiliate networks will really help you to find your favorite affiliate program fast. You can join one or more of these affiliate networks for free and then you search on their database to find an affiliate program related to your website or interests. I'd recommend ClickBank as it is the number one affiliate network for digital products and pays their affiliates every 15 days. You'll find affiliate programs that pay from %50 to%75 for every sell.
Other good affiliate network is Amazon, where you'll find thousands of very good physical products and books to promote. They pay you %10 commissions and will give you an affiliate link for every product in their database. LinkConnector and CommissionJunctions are also worth looking.