where to get opening theory

i have recently started chess serously and my coach told me to learn some openingss,whats a good place for that(or book)

Nevermind if openings are useful, but thats the stuff he should tell you or provide

I think what you need first is to have an overview of the main openings so you can chose which openings suit you.
I like Reuben's Fine book Ideas behind chess openings, but it may not be easy to find.
It's outdated in a way, but you don't read this book to study specific lines but to understand what each side is trying to achieve in different openings.
I think it helps have a better understanding of what chess is about, and it should help you chose the openings you may enjoy.
I don't know of any recent book that does that.

What chess is about? That reminds of the movie were they send romans to disover whatjesus is about, and after thhey discover its about finding god witghin yourself they get killed when they come back.

Develop your pieces, get your king to safety and fight for the central squares. Do not memorize openings move by move.