There are free alternatives for sure. Lots of great and not so great videos & postings all around the web. Nonetheless, after watching literally everything the web had to offer, I paid for a CGC subscription to really get started with Resolve. The main advantage over the free stuff is that you get the complete package and have it available at one place. No searching the web for information on certain tools or aspects of color grading, CGC has it all covered. In case you just wanna learn the software from scratch, it offers an easy to follow structure to do so step by step. The best part for me is that as a subscriber you can download the tutorial videos (so you can watch them after your subscription has expired, you then just won't get the new stuff that is added on a regular basis) and CGC's Denver Riddle is available to you if you have any questions or need further assistance. Wrote him many times, always got a helpful response within one workday.
I think the same goes for similar online tutorials offered by color grading veterans. I'd totally recommend investing in one of those professional programs if you're new to Resolve. CGC was my pick and I can totally recommend it to any DaVinci Resolve newbie that really wants to understand every aspect of this powerful piece of software.

Also saves a lot of time you'd spend searching the web for information - in my eyes that alone justifies spending the money.