most frequently using queries to organize data for visualization

I am new to this Power BI. My Management asked me work on PBI Reports. I have referred few Community Videos on Youtube and played with some sample data. I can say, this is a Most Power Tool. 

I have to work on large database (Oralce). It contains more than 40 tables. Can any one guide, what are the most using queries by you. Because, I am not familiar with advanced queries.

Please suggest me some tutorials and links in terms of different scenarious. So, I can get hand on the Queries stuff.

I guess, I have explained well. If not, please ask me. So, I will try to provide more information.