You may well know about Google Adwords Keyword tool. It's one of the most effective keyword research tools out there, and it comes completely free. The tool shows the number of searches done on Google in the past month and approximate average number of searches for each suggested keyword.
the more people search certain keyword in google, the more potentially effective it is. use google adwords.
The AdWords keyword tool is a good place to start. But in order to refine and optimize according to your specific business and website - a running campaign is needed. After launching - review your search term performance and re-construct the campaign accordingly (add negative keywords, more specific ad groups, ad copy etc) Important: avoid mixing Google's Search and Display networks in one campaign.