Selecting an affilate program is the most important step of establishing your business, as no amount of effort can make a bad program work for you.  To begin, you'll need some candidate affiliate programs.  Try searching Google for '"affiliate program" keyword', where keyword would be a one of the major keywords used on your site.  You can also find out about affiliate programs through forums or online communities.

When deciding on an affiliate program, the first step is to be sure that you understand a potential affiliate program's terms and conditions.  Not all programs are worth the time and effort, as some are designed to expect unrealistic sales goals before receiving a payment.  Make sure you understand exactly what the requirements are to be paid by your program before promoting it.

Secondly, make sure that the affiliate products you are promoting are highly relevant to your website content. Visitors clicked on your website because they were interested in the content of your site and, if your products are relevant to them, they will be more likely to buy the affiliate products you are recommending. 

Thirdly, make sure the products are a good value to your site's visitors.  If the product you are selling is not worth the price or is not a valuable product, you lose credibility.  Even if you make money on a small number of sales, you can lose credibility and discourage new visitors from taking your site and its content seriously.  For example, if you use your blog to promote a bad affiliate program, readers will doubt your credibility and may stop visiting altogether.

Another factor is need.  Any product that appeals to something that an individual has a need for, such as food or natural gas service, is more likely to sell than something an individual just has a desire for.  

The more useful the product, the more frequently it can be consumed, and the more related it is to everyday life, the more potential sales are available.  The best-selling affiliate programs are those that sell products that the majority of people need.  Think about the target customer for your product; if 50% of your website's visitors are potential customers, the product will sell far better than if the product would only be marketable to 5%. 

One very positive aspect of affiliate programs to look for is the "life-time cookie".  A life-time cookie means that you will get paid for referring a client who makes a purchase through your affiliate link even if they buy at any point in the future. A actually good affiliate program will offer life-time cookies, which will create a long-term revenue stream for you that does not require constant new sales.