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Deciding on the right keywords can be a difficult task. I agree with other comments that Google Adwords Keyword tool is the best place to start. Remember that the best approach is to place the most important keywords or a phrase in page title, h1 heading, description meta tag, and in the top paragraph content. Here is an article on keyword density and consistency that goes over Adwords tool and provides more insights on how to best select keywords. Hope this helps.
The best place to start is probably the Google keyword suggestion tool. It gives you options to find keywords based on your website url, category, and/or words and phrases. It is free, which is nice, but more importantly, who better to guide you in finding keywords than the company that matters most when it comes to search results.