Step 1: Choose a affiliate company... make sure you can work with them
ClickBank - Became our favorite one. Easy, reliable, good service, worthwhile products...etc. Plenty of stuff to choose from, and you are all set to advertise it on your website, blog etc.
Chitika - eMiniMalls service, is excellent choice for existing website owners or bloggers with a good history. They let you use a keyword, and the MiniMall than shows only relevant products.... Great for your viewers to spend their money on.

Step 2: Promote a program ... make sure it is a reputable one!
Internet is like the real world, you can only blow your reputation once, and people will leave you. So you are better to check the affiliate program you are about to join. Also get data about the conversion, so you can calculate that you make more money with the affiliate, than what you spend on developing the site, etc.

Step 3: Promote something you are knowledgeable about... and probably not only promote one thing... find more than one affiliate partner int he same niche!
Multiple possibilities look better for your visitors, you can compare the results. Variety of income source.

Step 4: Article Marketing... great way to get your site or blog known!

You do not get paid for the article itself, you still make money with your site and the affiliate products you are promoting. However, with articles you plant your seeds in many places, making it easier for the people to find your site or blog.

Plenty of article directories are out there to choose from. Make sure you read their policies first, harder to correct a refused application, than to do it right for the first time!

1-Article Submission Sites Master List
2-Top 50 Article Directories By Traffic, Pagerank