Writing is hard graft; getting published can be even harder. Book publishers and agents are inundated with manuscripts. They receive hundreds (if not thousands) each week. So, when your book lands on a publisher or agent’s desk they are looking for a reason to reject it. Rejection is easy, just a standard letter and the book is no longer their problem.
The book industry moves at three speeds: slow, dead slow and stop. If you are writing to be topical now e.g. the next Harry Potter, by the time you've finished the novel, or even if it is accepted, Harry Potter will be history. Don't write to be popular, write what you want to write, and often what you enjoy reading.
You might be an excellent writer with an excellent book proposal or manuscript but that won't guarantee you publication. Books are published and sold as 'categories' ( look in any bookshop or on line bookstore) so your book needs to fit into a 'category.' It also has to 'fit the list' of that publisher or imprint i.e. the style and type of book they publish. Then it has to stand out in that list enough for it to be sold into the trade (booksellers and libraries) in order for the publisher to make a profit. And an instant profit at that! Gone are the days when publishers used to nurture talent over several books before an author had a breakthrough novel. That doesn't mean to say that publishers always get it right - they don't. They might think they have spotted 'talent' but the book can still fail to 'sell through.' This can be because of lack of marketing, (which is often the case and is why authors need to do as much marketing as they can themselves) but it can also be because the publisher has misjudged the market. And if your book is published but fails to sell its expected quantity then you will be dropped even though there is nothing wrong with your writing or your book.
Most book deals are made with authors the publisher and/or agent already knows, proven writers make their life simpler. How to get known and break in to this circle is the difficult part. Approaching and getting published by one of the smaller publishers might help you start to build a track record and learn more about the industry. Small publishers can also be very innovative - they have to be to make enough money to survive. But you will need to market yourself. The same goes if you decide to self publish. Then you will also need your product (book) to be professionally produced, which means professionally edited and typeset, with a stunning eye catching and relevant jacket, priced to suit the market, printed by a respected and professional book printer and the book the right size for its 'category'.
Most books sell less than a thousand copies. Most writers make almost no money. If you are in it for the money then think again. There are easier ways to make your fortune. Writing, like getting published, is hard graft but if you love writing then write on...and enjoy!