YC Startup School puts out some excellent content. I recommend the whole series but in particular "How to Get and Evaluate Startup Ideas" [1] and "How To Talk To Users" [2].

[1]: https://www.youtube.com/watch?v=Th8JoIan4dg&list=PLQ-uHSnFig...

[2]: https://www.youtube.com/watch?v=z1iF1c8w5Lg&list=PLQ-uHSnFig...