There are many SEO tools that let you do all of that. The one that I know are BuzzSumo, SEM Rush, SpyFu, Ahrefs’ Keywords Explorer, most of these are paid ones but you can also use Alexa.com for free to check out competitors and which keywords get them the most traffic.
If you are familiar with a niche and you sit down and think a bit, you will come up with different keywords that might be targeted in that niche.