The classic answer has always been 'use Google Keyword Planner', but don't it is flawed. This video outlines why:
A general approach to keyword research is here:
But instead of using keyword planner use something like Ahref Keyword Explorer (it isn't free):
A good free tool is the Keyword Shi*ter here:
You need to go through this yourself as it scrapes Google quick answer queries and can come back with a lot of rubbish. Also, it doesn't provide search volumes. I use this a lot.
The best data to use is Google Search Console data or run a small paid search campaign to test keywords as this data is proprietary and unique to your website.