There are hundreds of sites claiming to offer all kinds of SEO assistance, but not a lot of them are actually giving valuable information.
I put together a list of ones I have personally used and consider to be legitimate sources of information. Feel free to add ones you like in the comments section.
https://www.semrush.com/ – lots of legitimate SEO information. I found this to be exceptionally powerful to find and gauge your competitors and keywords they are targeting.
https://www.woorank.com/ – not so powerful on keyword and link research as the sites above, but this one gives quick overview and valuable suggestions for on-page SEO improvements.