Well "ahrefs" is the best yet "paid tool" available out there, you can find out relevant search volume on a specific keyword. On other side I'd recommend you using medium tail keywords for better traffic, for instance: all your service is about "web development" then you might use "professional web development services in US", though such keyword captures wider audience and take less time to rank your site.

Old school suggests that your content and writing will suggest the best "keywords".

If, on the other hand, you are looking for "keywords" to fool users you have the goods open up a dictionary and go from there. Not kidding.

One of my most popular pages on a rather obscure niche site that is hit by EXTRAORDINARY NUMBERS (and amazingly prolonged visit time) is a list of 17,000+ words and the number frequency of each which appeared in 11 novels by a particular author. An alphabetical list of words. Nothing but a list, with two paragraphs describing what that list was about.

Meanwhile, articles about that same author and those novels ALSO had great numbers ... because those same words were in there, too.

Which suggests to me that some searches are pretty brain dead and you can't figure out everything!

HOWEVER, if you are in the business of ad displays as a revenue stream, then that "dictionary" becomes a "phrase book" and that's a different kettle of fish. A whole cottage industry has grown out of that and as noted above, there are sources out there ... but no hard and fast rules (for success).

Yet "keywords" have no value unless they are part of CONTENT ... and g, these days, is into RELEVANCE and CONTENT ... not so much "keywords" in their lonesome.

Have the content/purpose/answer and those keywords will be gold ... if you lack it you get one hit and gone---and no conversions.

There are lots of different ways to find out the best keywords for your website. Many tools are paid tools and many are free too. Although paid tools work better in many cases, free tools are not also bad at all. I would highly recommend you to use Google Keyword Planner to find out the desired keywords for your site. It is Free to use and can really bring some good results if you know how to use it.

1. Brainstorm - Think about what users looking for a site like yours would type into Google. Using a marketing persona can help here as well as looking at related industries. This will help you put together a list of seed keywords.
2. Keyword Tools - Input this list into Google Keyword Planner, Google Trends, KeywordTool.io, Quora, and other places to find related keywords. You can also check Google Search Console and related search terms at the bottom of SERPs for terms people are looking for in relation with your keyphrases.
3. Evaluate commercial intent - This will help you identify where people are in the buyer's journey. The further along, the more likely they will convert.
4. Analyze the competition - In the final step, you need to figure out for which of these terms you can realistically rank in the top 10 of Google. Otherwise, optimizing your content for them might be a waste of time.

The above is basic keyword research. However, be aware that, by now, Google is much better at understanding the intent behind search inquiries and judges web content based on that. For that reason, focusing on search intent is a much better idea than pure keyword optimization.

Hope this helps!

After running Adwords for a while I managed to see what users type in when they search, and surprisingly it is not what I would expect. For example, If I am looking for anything, my search string narrows it down perfectly. But when I check the most commonly used search strings for similar results, what they typed in makes no sense, almost like the sort of gibberish that one might expect when using google translate. Perhaps that is what they doing... searching in English after translating their own language to English?