Trying to help my parents start investing – where should I learn first?

My parents are in a position where they could invest around €1,000 a month without it affecting their lifestyle, but they have basically zero knowledge about investing and don’t know where to start. I really want to help guide them so they can build something solid and retire comfortably.
The thing is I’m still learning myself. I don’t want to give them bad advice or miss something important.
So I’m trying to figure out:
Where can I properly learn about investing (stocks, ETFs, long-term strategies, etc.)?
What are the most important basics I should understand before helping them?
Any beginner-friendly resources (books, YouTube channels, courses, etc.)?
General tips for someone investing monthly for the long term?
I’m not looking how to get rich quick stuff just solid and realistic strategies that make sense over time.