I use the Google Keyword Planner. I will also type searches into Google and look at the "Searches related to" portion of the search results.
Hello, I can say, that keywords are very important as for your traffic as for your google rank too! If you need a good and free keywords you can use Planner, Google AdWords!
Just go straight to http://RankBrain.me to do your keyword research for free. The tool gives a lot more relevant keywords than google keyword planner. This will save you a lot of time.