It’s a broad question, but the absolute first place I would start would be with this course:
and then move on to EDNA Expert @Greg 's wonderful series on Power BI best practices. These parts of his five part series address data prep/data shaping/data modeling, and I think will put you on an excellent path for what I think is the most important aspect of Power BI. If you get your data well modeled according to best practices, most DAX becomes relatively simple, while if you get the modeling wrong, all the DAX expertise in the world isn’t going to save you.
I hope this is helpful.
- Brian