Forum Discussion
Good Source to Learn DAX
There are lots of sources
Start here with guided learning
https://powerbi.microsoft.com/en-us/learning/
MS have started a new learning site. Its new so there isn't much yet, but they are adding more.
https://docs.microsoft.com/en-us/learn/browse/?products=bizapps-power-bi
There is a free edx course
https://courses.edx.org/courses/course-v1:Microsoft+DAT207x+1T2017/course/
Curbal is a fantastic chanel ruthpozuelo has loads of videos on lots of Dax functions.
https://www.youtube.com/channel/UCJ7UhloHSA4wAqPzyi6TOkw
Powerbi tips have a good set of links to sites including learning
I recommend this book on Power Query; excellent writing and helps you build your understanding on the whole system for which you need DAX. It's not just for beginners, and it takes you to an advanced level you WILL need.
M Is for (Data) Monkey: A Guide to the M Language in Excel Power Query
Then you could read this one.
Learn to Write DAX: A practical guide to learning Power Pivot for Excel and Power BI