As I add posts about this site, I'll presume you're already familiar with the basics of Python, Django and Wagtail.
In case you're starting out, I would learn those in that order.
It's hard to beat Real Python for everything from beginner to advanced topics ranging from coding fundamentals to web development, machine learning and more.
Python Tutorials – Real Python
Learn Python online: Python tutorials for developers of all skill levels, Python books and courses, Python news, code examples, articles, and more.
As for Python, there is a ton of resources out there for learning all there is about this Python-based web platform.
The Django Book is the full documentation from the Django Project itself and kept up-to-date. There's not much about Django you can't learn from here.
Mozilla's Developer's site also has a very comprehensive site, including free coursework for beginners:
Django Web Framework (Python) - Learn web development | MDN
Django is an extremely popular and fully featured server-side web framework, written in Python. This module shows you why Django is one of the most popular web server frameworks, how to set up a development environment, and how to start using it to create
For more bite-sized lessons, try Django Girls and How To Tango With Django - the latter is good but a little dated now.
There are much fewer resources for Wagtail, but don't let that put you off. The benefits of using Wagtail far outweigh the lack of material out there. Once you see Wagtail's streamfields in action, you'll never look at Django again (almost).
Maybe start out with an introduction at Wagtail's own site and Made with Wagtail for some demonstration sites.
Learn Wagtail has an excellent set of free and paid-for (for a very low fee) instructional videos that'll take you from beginnings to early-advanced topics. I would say start here. You can maybe skim the Headless CMS and PET videos if you're pressed for time.
Finally, you'll find a very active community of Wagtail developers over on Slack, Github and StackOverflow.