I want to give my impression about the prologue, which one 'the starting is good' or 'the start is good' is the correct way?
Choosing between both options, the better answer would be "the start is good."
This is because you're talking about an object, the prologue, and not an action. However, "the start is good," sounds a little unnatural and isn't used by very many native English speakers.
Instead, I'd recommend something along the lines of, "the beginning is good," or instead of using words like start or beginning, you have the option of mentioning the prologue as-is (e.g. "the prologue is good."), unless you're talking about just a section of the prologue.
I'd be more likely to say
the first part is good (but then it goes downhill)
"Starting" is not idiomatic. "Starting" actually means "the process of beginning something" and not "the beginning part of [something]"
Starting a new job is always exciting, but can be challenging.
As J.R. points out in his comment, you can say:
It starts off good (but then gets boring).