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) SEMrush - I believe that among all the 3rd party software, SEMrush has the largest keyword database. Their search volume data is pretty accurate and aligns with the Google keyword planner. Also, based on the type of content that needs to be produced (i.e. informational, transactional, etc.), one can utilize different filtering options available in it.
You can also filter by query, which is useful when looking at branded queries, or when looking at specific words. For example, only show keywords that include the term "SEO". The graph also allows you to spot trends in across the available metrics and compare week-on-week or month-on-month. This can help you to drill down and monitor progression over time, allowing you to answer questions like "have my branded keywords received more clicks in the last month compared to the previous month?", "has the CTR improved?", "did average positions in Google improve?".
If you're a regular blogger, these are probably the topics you blog about most frequently. Or perhaps they're the topics that come up the most in sales conversations. Put yourself in the shoes of your buyer personas -- what types of topics would your target audience search that you'd want your business to get found for? If you were a company like HubSpot, for example -- selling marketing software (which happens to have some awesome SEO tools ... but I digress ;-) -- you might have general topic buckets like "inbound marketing," "blogging," "email marketing," "lead generation," "SEO," "social media," "marketing analytics," and "marketing automation."
What I really like about them is extensive detail including the keyword difficulty. Ahrefs uses clickstream data to also show how many clicks you will get from the search engine. This is very useful after knowledge graph integration, as many keywords may have huge traffic but they hardly get any clicks from the search engine. Reason being, they get answers directly from the Google search result. Example of one such query is: “birthdate of any celebrity”
We hope you found some of these tools useful. It's worth noting that if you don't have any content that suits a particular keyword, you can always create it. Just remember to make sure that the content answers the search query that you're trying to target. For example, somebody searching for the term 'Log Cabins Phoenix' is not going to be satisfied with a page that offers log cabins for sale in New York.
In addition, you can dig into the paid side of search and find out what keywords your competitors are bidding on, and then leverage those keywords for your own organic benefit if you're not already doing so. Search Metrics does this as well, but I've found SEMRush to provide a greater range of keywords and they save more historical keyword data than Search Metrics.
I want to share how and why I sold the company in order to both educate and inspire potential software entrepreneurs. I certainly don't know everything and obviously companies sell for much more than I sold Long Tail Pro for (I won't be featured in Tech Crunch anytime soon); however, I'm willing to share what I have learned and hopefully that can be beneficial to a few of you.