Brian Dean is a really good reference when you are just starting to learn SEO. He has tons of high quality posts and very actionable.
I personally use Ahrefs for everything SEO related actually: KW research, competitor analysis, content research, ... They have improved their tools the last 2 years a lot. Best on the market right now imo, but yeah kinda pricy. Altough if you are really serious with your business you need something like this: you have to be able to do a competitor analysis, have a good mapping of your KW, start generating some content, ...
I know there were some sites were you could share ahrefs accounts (this is against TOS of course), but not sure if they still exist.