Check out Video #5 of Power BI Series and learn about DAX Measures and Slicers in Power BI Desktop.

This video covers:

• How to create a Measure
• Intro to Quick Measures and standard calculations
• Intro to Slicers and Edit Interactions

If you are starting learning DAX I suggest:

• Book:  Power Pivot and Power BI by Rob Collin & Avi Singh.  This was one of the first books I read about DAX.
• Blog:  DAX Fridays  by Curbal.com . Ruth releases a video every Friday covering DAX functions (and more) and to date, the playlist counts with more than 80 videos.  Awesome work .

There are plenty of other sources (books, blogs, videos, etc), but these are my recommendations if you are new to DAX and Power BI and want to learn the basics.