There are different approaches to researching keywords. Tools like Wordtracker can cost a reasonable amount of money, but paying for a subscription to a service like this enables you to find some of the more obscure keywords. You should also consider free tools like SerpStat, which can provide you with question based keyword ideas; a very useful source of possible longtail keywords. Don’t be afraid to create a massive list of keywords. You should be adding content on a regular basis, and when mapping keywords, you can assign some of these to your regular blog posts, to your social media posts, and some to your main pages.