The forums are an area for discussion about specific features, problems, or concepts in Power BI.
The only appropriate response to such a broad question is a suggestion to google for "Power BI tutorial" to learn more about the products and how to do things. Additionally, it might be useful to peruse the documentation for the items which can directly address your questions.
Edit: Perhaps you might find the Microsoft-developed Power BI edX course helpful.
In general, you'll have to use Power Query to get data.
You mentioned YTD and prior year measures. The DAX functions which will support this are TOTALYTD() and CALCULATE() combined with SAMEPERIODLASTYEAR(). These all require a date dimension. Google for "Power Pivot date dimension" to understand the purpose, requirements, and usefulness of a date dimension.
If you run into specific problems in this process, feel free to start a new thread about that topic to get more focused help. As it stands though, the answer to your question *is* the product documentation and several already published tutorials.
