Most of the time I use the Google Adwords Keywords Tool which can be found by typing "google keyword tool" into the search bar. It can be rather challenging to use if it is your first time and will require some practice. Read the informative article in the learning center for some more about this.
I find that I also use the google search bar a lot. Typically, I will enter the start of a phrase and see what google tries to finish saying. These will indicate some of the most often searched topics. This isn't a fool proof plan, but does give a rough idea on where to start.
The final way I go about it is by just writing. I find that if I truly enjoy what it is I am talking about, the more satisfaction I get from it overall. Traffic can be adjusted afterwards if it turns out to be subpar. Tweaking a writing you enjoyed doing is much more rewarding than putting effort into something your forcing yourself to write. Just my thoughts though. Good luck!