Nowadays, there are a lot of resources you can learn Power BI, with most of them being free so there is no reason to pay unless it is something that is very specific that you are looking for, but always check for free alternatives as most of the answers are out there. Depending from your learning style (whether you prefer reading or video presentation), always make sure you follow along as the best way to learn is by practice.
This will be a short post with the sources from where you can learn Power BI, sources which helped me a lot along the way.
1. The official Learning Site from Microsoft - Learning | Microsoft Power BI
2. The official documentation from Microsoft - Power BI Documentation
3. All white papers for Power BI - Whitepapers for Power BI
4. Official Pathway for Exam Da-100 - Exam DA-100
5. Guy in a Cube - Free YouTube videos
6. Havens Consulting - Free YouTube videos
7. Curbal - Free YouTube videos
8. Entreprise DNA - Free YouTube videos
9. What am I doing here?!
10. There are some really good courses in Udemy, some of them for free or the ones that are usally around $10. Do not ever pay more than $10 per course there as all of them are in sale, if course you like is not for sale then go a quick google search for coupons.
This was a quick list from where you can learn Power BI, in the meantime I might be updating this list so stay tuned.
Cheers!