Just to throw one more resource on the stack that @Melissa recommended, I would also suggest taking a look at the DAX Patterns - Fix Incorrect Totals section compiled by eDNA expert @Greg:
In addition to Greg’s excellent overview, and many of the videos Melissa recommended, it also has links to other related forum posts. This DAX Patterns section is a great resource to check when you run into problems, since Greg has developed patterns for the most common issues/questions we see on the forum, and is regularly adding new ones:
I hope this is helpful.
- Brian