I have filled forty-one notebooks since I started. Most of what is inside is useless: shopping lists, train times, half sentences I cannot read anymore.
But every so often I open an old one and find a line that changed the next year of my life. You cannot know in advance which line that will be.
No system
I have tried every method. Bullet journals, index pages, colour codes. None of them lasted. What lasted was carrying the notebook everywhere and writing in it without caring how it looked.
If you want to start, buy a cheap one. Expensive notebooks make you afraid to write badly, and writing badly is most of the work.