DATA MODELLING

Have you tried googling?

https://docs.microsoft.com/en-us/power-bi/guided-learning/modeling

https://www.tutorialspoint.com/power_bi/power_bi_data_modeling.htm

https://www.sqlbi.com/p/data-modeling-for-power-bi-video-course/ (expensive but these guys are great)

I would start on this site, up at the top, Learn | Guided Learning. Also, try https://aka.ms/diad.