This is a good question.

Article marketing is a very powerful way to promote products and services and will ultimately allow you to have a long-term marketing machine in place. Article marketing becomes the most profitable when you have something to market that can generate income or sales for you.

The quickest way to find a product to promote is to go to Clickbank and search for a product that is in the niche you are interested in. I would first pick a niche you are interested in because it is easier to write articles about things you are interested in. Then in Clickbank, choose about 3 different products in your niche and promote them. Do some keyword research to determine the search terms that people are using to find the product you want to pick and then write your article with those keyword phrases included in them.

Once you have the system down with Clickbank products, you can begin to do a search on the Internet by typing into the search engines the following phrase: "affiliate program" +
You will receive a list of sites that offer affiliate programs in your niche.
To make it worth your while, pick one that allows you to earn $20 per sale.

This article marketing resource also has a great section on how to pick products to market using articles.

Once you have written your article, you can promote your article to article directories that rank well in the search engines. This article submission services does a good job at submitting your article to those search engines and will submit your articles to the higher quality article directories as well. Articles submitted by this article submission service tend to be higher quality and will be more likely to be accepted by the different article directory and e-zine publishers.