When someone uses Google to search for particular content, they use specific search terms to find what they’re looking for. You should think about which search terms you want your website to be found under and use these as your keywords.
You might have noticed that when you search for something on Google, it offers you suggestions to complete your search. Take a close look at these suggestions and use them to gather ideas for your keywords.
With every search, Google also shows you related searches underneath the search results. You could also use these as ideas for your own keywords.
The Keyword Planner is a free tool from Google AdWords, which you can use to search out keywords for your website and analyze the effect of different keywords. To use the Keyword Planner you just need a Google AdWords account.