The Commonplace Book - get started!
What in the world is a Commonplace Book? It is simply to compile knowledge by writing information into a book. This can be ideas, quotes, anecdotes, observations, poems, charts, etc. People have been doing this for years but I think the practice has gotten lost in our age of technology and busyness.
When reading a book, do you every come across a fantastic quote? Is there a poem you wish you could have at your fingertips? How about a verse or something from Shakespeare? All of these things can be written into your commonplace book for you to "own" and be able to reference.
The act of writing is important because you actually have to use more effort than just taking a picture or typing something. It cements it better into your brain and helps when it is time to recall it.
You can have a different book for each type of notes you take, but the traditional commonplace books had all the thoughts and ideas in just one book. I have tried to use fancy, leather notebooks and found that I just got caught up in the "it better be profound and look awesome" trap. If this is the case for you, consider using a spiral notebook or a composition book. You can even write things on 3x5 cards and tape them into your notebook.
I also really like the travelers notebooks because they allow you to have a variety of smaller notebooks in it and those books are a lovely size. (Not to mention, they feel awesome in your hands!)
Whatever you use, the idea is to get started. Keep it simple, and have fun collecting all those great thoughts and ideas. You will be glad you did!