Find out how people would search Google for your offer. You can start by using the Google Adwords keyword research tool to find how many people actually search for specific terms.
I think Google Keyword Planner is one of the best free tools to do an extensive keyword research for your website. Keyword research is very important to find the right keywords to rank them in various search engines.
Keyword Research is one of the most important thing in SEO, if you selected the wrong keyword you can't rank your website in Google or you can't get the required ROI. Google keyword planner is one of the best tool for Keyword research.