How do I publish my book?

 

If you're seeking to get published in the traditional sense (i.e., you want your book printed by one of the six major publishing houses and stocked in bookshops), you need to find a literary agent. It's very uncommon nowadays for a publisher to take an unsolicited manuscript for consideration, and the only way in is through having an agent. 
To get an agent, you need to find a selection of agencies that specialise in your genre and submit to them. This involves sending them a query letter (including a brief synopsis), sometimes a detailed summary of your book (no more than a page) and the first three chapters of your work. Different agents give various instructions, so be cautious when submitting.
Getting into the publishing industry through this route is extremely difficult, and you mustn't feel disheartened if you're rejected. I've submitted two books to over one-hundred agencies combined, and had no luck, but that's not to say it won't happen for you. Publishing is a very subjective business and all it takes is one person to say yes. 
The other problem with taking this route is the time span. If you think you'll submit your book, land an agent and be published in a few months, you're very wrong. 
Finding an agent alone can take years, and it all depends on how much time you're willing to sacrifice. After submitting to an agent, the wait time can be up to or over three months, and in the off-chance you do gain representation, you're then looking up to many more months of submissions to publishing houses. Once signed with a publisher, your book will be on the shelves within a year. 
Agencies and publishers are sent thousands of submissions a month, so you have to make sure your book is a) edited to the best of your ability, b) stands out from the rest and c) is what the current market is looking for. 

 

If you're looking to be published quicker, or you've simply given up with the agonising wait for an agent, self-publishing is the next best thing. I know a lot of people turn their noses up at this route, which is understandable considering the amount of un-edited, un-structured eBooks online, but there are a few diamonds in the rough out there. The key is to not be put off by the label 'self-published', you aren't painted with the same brush; no two writer's are the same. Self-publishing doesn't necessarily mean uploading your book to Amazon and having no interest, or finding success similar to Fifty Shades of Grey, it allows you to market yourself the way you like. With this method, you can design and make your own book cover, market yourself and post book teasers via social media. The problem is that these things can cost a lot of money. You have to spend money to make money, or have friends who can help/specialise in photography, graphic design, editing, etc. Getting word out off your own back is hard to do, so you have to make sure to promote yourself every chance you get. That means making promo posters, bookmarks and swag all by yourself to put out there.
Stemming off from that, there are numerous independent publishing houses around the globe that specialise in all genres. I'm signed with one in America and they're fabulous at what they do. They help share your work around the online community, make you a cover (I used outside sources for mine as I wasn't signed with them at the time), edit your book, and support you on your journey as an author. They don't charge but take a little percentage of your sales which is very fair.