Depending on your topic / vertical and your geographic location the search engines may have vastly different search volumes. The tool can only possibly offer approximations. Exact search volumes are hard to find due to vanity searches, click bots, rank checkers, and other forms of automated traffic. Exceptionally valuable search terms may show far greater volume than they actually have due to various competitive commercial forces inflating search volumes due to automated search traffic.
If you're struggling to think of more keywords people might be searching about a specific topic, go to Google.com and take a look at the related search terms that appear when you plug in a keyword. When you type in your phrase and scroll to the bottom of Google's results, you'll notice some suggestions for searches related to your original input. These keywords can spark ideas for other keywords you may want to take into consideration.
The simplest KW research tool is google. Type in your topic and scroll to the bottom to see what related results show up. Use those to either write an article or to supplement other articles with some of the keywords. Yes, they are primary phrases so you may not rank for them right away but building your site with these in it will help get rank for them eventually. Also look at what other articles that show up for the keyword use.
1. Create a New Project. By clicking on the “+” icon located at the left upper corner, a new pane will show up allowing you to: enter your new project Title; choose your project’s Country (which affects the total of local monthly searches and top 10 Google results shown); pick your project’s Language (which affects generated keywords and the top 10 Google results for users that use that language); select your Search Network (which affects the number of reported searches depending on your choice between “Google” or “Google & search partners”); and toggle the Adult Ideas switch on or off.
I just have the free version right now so I don't know all that the pro one can do. But even the free version has A LOT of tools you can use, I haven't even figured them all out yet. But one that I have used is their Content Optimizer. You can take a new or existing content piece of yours, and compare it to one of your competitor's pieces on a similar topic, and see where you might be lacking based on the keywords that are used in each piece.
It arranges your search volume by separating the relevant keywords from those considered irrelevant. It presents those keywords, which it believes would help rank your search engine very high. Once you indicate that you want your keywords to be filtered, the tool would help to arrange that. You know that is very important for your Adsense and adwords campaign. The program is relevant for your CPC and other internet marketing strategies that you want to use.
3. Finally, there's just good old research through trends and news. Google Trends, keeping up on industry news of the business, and even newsjacking (if there are relevant topics). These all require different resources depending on the business, but once you find the leaders in their news you can not only leverage them for keyword research but also glean insights into how you can become an industry leader yourself (and dominate SEO).