What does the quote "today a reader, tomorrow a leader" mean, and what could i write an essay about it on?

Basically, it's similar to "the student becomes the teacher". One day, you'll start out learning how to write essays, and in the future, you might teach others how to do it. It means that the more you learn and experience, the more knowledge you'll be able to pass onto someone else.

You could write an essay that argues that the more you learn and experience, the more you'll have to teach, and include examples of people who contributed to accomplishments in history and that they too began as a "learner" and not the teacher or the master they're considered.

with whom will you be coming to the party? this is written correctly.

the answer is a because the astrophel does more and sidney goes to the astrophel more than stella.