Howdy!
The book that you are using (Statistical Rethinking) is in my opinion, the best book for starting from scratch as a beginner to Bayesian modeling. I really like his free lectures on YouTube as well.
In addition, I think a great way for a beginner to start is by using the brms package, which uses Stan on the backend. It is extremely flexible and is also very user friendly, well-documented, and has numerous examples and vignettes. It has it’s own tag on this forum.
In addition, Solomon Kurz has put together a wonderful resource where he has worked through the Statistical Rethinking book using brms, rather than the rethinking package.
Good luck!
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