Starting out, you need only two affiliate programs, one to recommend physical products, the other for information products.
The clear winner for physical products is Amazon. For information products, the best one used to be ClickBank, but JVZoo has overtaken it this year.
Here's why Amazon and ClickBank are the two best options.
Amazon Associates
The Amazon Associates program is very well known. You use their links to promote certain products and you get a cut for every item bought through your links. Commission varies between 2% and 8%, depending on the product.
But every time someone lands on Amazon trough your affiliate link, you get a cut of everything they buy during that visit, not just the product you recommended.
This makes using the Amazon Affiliates program really powerful.