New to SEO - Need Help from the community.

I am wanting to learn SEO for my online business. I already have a little idea about it and know a few things here and there. What is the best place to learn SEO in a way that is practical and I can implement it on my website or with my content etc. I have the following questions?

1. What is the purpose of using Headings while writing descriptions for my e-commerce website? How do I know where to use Heading 1, Heading 2, Heading 3 etc?
2. Is Yoast SEO (the free plugin) really useful or not? (I want to know so that I understand whether time should be spent on this or no).
3. What is the most effective way to do keyword research using free online tools? (We are a new startup and cannot afford paid tools currently?)