Imagine a green room.
The room that takes shape in your mind is going to depend on who you are, the things you experienced and how you process information.
What kind of room did you imagine? Was it a room you've been in that is actually green? Is it a room that you've been in that isn't green but has now turned green in your imagination? Was it the room you're in now? Was it a child's room? A living room? What was in the room that you saw? Was it just a featureless room with green walls? Was it an actual Green Room that you may have been waiting in before performing? Did anybody appear in the room you imagined? Were they doing anything? Did you know them? Was it in a house? An office building?
When prompted with something as general as, "a green room," the details that a person's mind fills in (or fails to) tells you as much about them as you can imagine.
Each Tarot Trump is a general prompt like this. Each Trump is a mirror that reflects both a part of every human soul and a part of the universe. So whether you know it or not, you already possess knowledge of the Tarot Trumps just having drawn breath as a human in this cosmos. You may not be able to articulate this knowledge yet but with a tiny bit of effort that can change quite quickly.
It is this effort that is required to read Tarot effectively.
Going numerically* it begins with the Fool which carries the number 0.
Even without seeing the card what does your mind conjure when you hold the ideas of, "beginning, "Fool," and "0," in your mind? If that is too complex (and it was for me at first) try holding each in your mind separately and gradually combine the ideas.
What beginnings have you experienced or witnessed?
When have you been foolish or seen foolish behavior?
When have you had nothing or have had it all taken away?
When have you had to start over after losing everything for being foolish?
Your answers to these questions will guide your understanding of this card. The meaning you read in the book or pamphlet regarding each card, the symbolism and art on it is a seed. Meditating on these seeds is what leads them to sprout.
Doing this exercise regularly with the Trumps is not required to read the Tarot but it really helps.
On keeping a Tarot diary. I do not do it but I know if I had the organizational skills and drive to keep one I believe it would prove enormously helpful.
*There are many ways to organize and and order the Tarot Trumps. Trying to find all the interesting ways to do so is an extremely instructive and rewarding exercise.