There are jillions of manuscripts, and only 24 hours in a day. Most publishers don't even look at unsolicited, unrepresented manuscripts. If you find one that does, your ms. will go into a 'slushpile,' it will be months and months and months before an intern-type looks at it, and it's then very hit-or-miss as to whether they will recognize the true worth of your effort.
So if you want to be published by a commercial house, have the book marketed, and thus having the potential to find an audience, you most likely need an agent. A real one, not a scammer. Real ones don't advertise, and they don't ask for $ up front.
So, how do you get one? It's a process, of course. Agents, after all, have the same problem as publishers--lots and lots of writers seeking publication. Here's a helpful online article. It's from the Science Fiction Writers of America, an author's organization. The info is perfectly good for non-SF authors.
http://www.sfwa.org/real/