There is a very active community around PowerBI with many sites specializing in different aspects of the product. The following 4 resources are ones I always fall back on recommending that provide both beginner and advanced users learning resources.
If you are starting to learn Power BI, there are several resources that will provide an excellent end-to-end introduction and also provide a community you can come back to.
The Basics – PowerBI Guided Learning
The Microsoft PowerBI site has a series of articles and videos that provide an end to end introduction. Power BI Guided Learning should be the first place you go.
News and New Features – The Power BI Blog
With the monthly release cadence of Power BI Desktop and services, the Microsoft’s Power BI Blog provides the latest release notes, patch notes, techniques, and highlights from the community.
Ask Questions – Power BI Online Community
The PowerBI Online Community provides a place where you can ask and answer questions in several forums, such as; Desktop, Services, Report Server, Mobile Apps among others. The site proves user questions and marked solutions.
DAX – Data Analysis Expressions
DAX is a library of functions and operators used to build formula and expressions that will augment your data model. These can be used in Microsoft SQL Server Analysis Services, Power Pivot in Excel, and Power BI Desktop. Two great introductions are provided in Dax Basics in Power BI Desktop and QuickStart: Learn DAX Basics in 30 Minutes is available on the Office support site.