We have already discussed the free Google keyword research tool in detail here.
All you need to follow this guide to help kickstart your SEO with zero investment.
Keyword research is a way to find the right and most searched keyword by users or they enter into search engines while looking for any topic.
Moreover, Defining the right keywords in content makes easy for search engines to display your article on the top results when people search for it.
10 Best Keyword Research Tool
Considering the Best Keyword Research Tool, Google Keyword Planner is the first.