If you’re asking what makes good music good, then the answer is dependent on the listener. Music in general can be summed up to a collection of sounds that have pitch. Pitch is the actual note delineated by the frequency of the sound. For instance most old office phones ring at A#. This would not generally be considered music but if you were to put it in a setting with other pitched sounds you can make music with it. Composer/pianist Glen Gould made music out of recorded monologues. He found rhythms and pitches there. So, in my opinion, music is anything that can be expressed using musical notation.
The first thing you, the performer/talent, have to do is work. Write, record your demos, and take on as many shows as possible.
Build a real following, on your own.
The time to seek out representation is when they start to seek you out.
They will see that you have developed some craft, and more importantly, a following.
If you have crap to offer to no real audience, then a manager is only taking your money. Some unscrupulous bastards are out there, though any real manager will not waste the time of taking 15-25% of nothing.
They cannot make you successful. They make it easier for you to become successful, by building and expanding upon the business plan you have set forth.
Are you a musician? Are you part of a band? What kind of product do you need managing?
Music managers are a dime a dozen and only a small percentage are worth their weight. The one thing any good manager will want is a good product. That means good music, a good look, a marketable image. Lots of music today is about image. Age and look play into it. Do you write your own songs? Are you looking for publishing or placement of music in commercials, film, TV etc. What is your niche?
There’s is so much you have to make a plan for where you want to go. A music manager will like a musician who has a plan and knows what they want to do with their career and is realistic about the road they must travel to get there.
Also you want to find management that has good connections for publishing and playing. Meaning good agents, good connections for radio, good connections with various resources that you need to propel your career forward.
Its a good place to start if you have your roadmap drawn.