Determining if there is sufficient potential traffic to bring business to your website or blog is key before you start a an online business.
Free keyword tools like the free version of WordTracker will tell you which keywords people are using every day that relate to your topic.
When you find a related search engine keyword, note its average daily search volume, but dig deeper. For example, if you discover a good volume keyword phrase related to birds, such as bird identification, dig deeper and continue using the tool to find phrases related to bird identification you may not otherwise consider.
Once you have a list of potential keywords for your free online business, search for those phrases to see how many results are being returned. I recommend using Google for this purpose as it is by far the most popular search engine used today and is likely to result in the most traffic to your site.
The keyword phrases that best combine the highest volume (popularity) with the fewest results returned in a search (competitiveness) are your best bet. Because your free online business will be starting small, you will not be able to rank well for search terms that return millions of results - those keyword phrases are just too competitive to waste your time targeting.
You know you've hit pay dirt when you combine high volume with very low competition.
An example I recently discovered was the phrase web seo ppc ny for which WordTracker estimated a daily volume of 1, 598 against Google competition of 90, 000. Examples like web seo ppc ny are definitely the exception and not the rule.