I want to start generating additional income streams and what I've heard of most is "passive income"! I googled a lot, but I am not quite sure what option fits a beginner best... Any advice?
It depends on your skills and your financial situation.
If you're an excellent writer then writing a book will likely be your best option. If you can't put 3 sentences together that make sense "write a book" is terrible advice.
Likewise if you don't have $100 to your name, "invest in dividend stocks" is terrible advice.
Youtube is a popular choice because you likely already have everything you need to start your channel. Writing is the same. If you really wanted to you could do both on your phone. You could also make a website, start a dropshipping business, make a course.....