SEO begins before you actually write an online article. Let say you want to write an article on cancer. Where do you begin it?Go to Google keyword tool and try to search keywords. You keyword research should be something like cancer in US, Colon Cancer, Breast cancer fatcs, cancer deaths in the world, stages of colon cancer etc.
You have to select keywords that get moderate hits (between 500 to 5000 searches in a month).Once you choose your target keywords, you have to develop contents based on those keywords.
Alternatively, you can also optimize your article for search engine by tweaking few things such as using keywords, changine title, adding fresh contents etc.