Option one is to seek publication with one of the major publishing houses such as Random House, Pan MacMillan, Penguin etc. Over the years the number of major publishers has dwindled or rather merged until there are now fewer of them, so the chances of getting a contract is limited. In order to get to first base you will either need an agent or a name, or even both. You are likely to get a better advance than with smaller publishing houses, and if you are fortunate enough to get a substantial advance then you might even get a sales and marketing budget behind your book. This means your book is more likely to generate sales (though not always) and most importantly for the publisher generate rights income from the sale of translation and other rights. If your book doesn't generate enough sales though you will probably find it nigh on impossible to get another contract for subsequent books.
Option two is getting published by an independent publisher. These are smaller publishing houses, publishing fewer titles and with much smaller and sometimes non existent marketing budgets. Their prints runs will be considerably smaller than the big boys mentioned above but they do provide a foot in the door for many new authors and steady sales (and income) for established ones. You will also find that they are more approachable and many will take unsolicited submissions direct, so it is not always necessary to have an agent. They also sell rights. In order to boost sales of your book you will need to undertake some marketing activity. In fact the more you can market yourself the better.
The third option is to go it alone and self-publish. Advances in print technology, the development of digital printing and the Internet have made self-publishing a viable option. This can provide you with a good opportunity to 'test' the market but you need to do your research beforehand. Make sure your product (the book) is of the best quality you can afford and that it can sit proudly on any book shop shelf. Research the size, style, type of paper and price point for your book by comparing it with others in the same genre or category. Set aside a budget for producing your book and one for marketing it. An effective marketing plan is essential as is an ISBN. Without an ISBN number your book will not be found by potential buyers. You can register with Amazon who will sell your book on line for you, but be prepared to give discounts to both on line booksellers, wholesalers and book shops, anything up to 55% is not unusual.
Another option is to produce your book as an ebook only. The advatange here is low cost. Nevertheless ensure it is professionally edited and the best you can produce. This is an emerging market and aside from offering your e book via your own web site, perhaps as a pdf, you can also offer it via Amazon through their Createspace programme. As with all the options mentioned above marketing your book is essential for generating sales.