Understanding keywords is essential to having success in driving traffic to your site.
The problem is usually that people see a hot topic and decide that is a great keyword. The most popular keywords are not always the best to use though. Sometimes it is better to dig a little deeper and find keywords that may not have the same volume of searches, and therefore do not have the high level of competition. It is very difficult to compete with keywords that get a million searches a month with the amount of other sites competing for the same keyword.
There are a lot of ways to go about digging up the best keywords. The manual process can be very time consuming. One of the tools I use is Micro Niche Finder (and yes that is an affiliate link).
Find what works for you best and stick with, but you can be sure that guessing is not the way to go.