The Hebrew letters are symbols, and like any symbol they reveal meaning in context. Think of it this way: what does the number one mean? What does the number one relate to? If you consider this, there is no "easy" answer, since you have a lifetime of using the number one symbolically and contextually. The Hebrew letters are the same: with use, you will start to understand them in the same way you use the letters of English or numbers. They are "simple" and sophisticated at the same time.
Regarding the collection of your questions, we can answer in the same way: how did you learn English or learn to use numbers? Slowly, gradually, but mostly by using it daily. So in the same way, be patient and work with the letters at the same time as the Tree of Life. As you see, they are interdependent. They need to be studied together.
Let me also say that you will not finish. Even after years of studying them, we still regularly discover new aspects and new meanings, and so continually feel like ignorant beginners!
A great place to center yourself in these studies is this book: