The first thing to do is consult a copy of the Writers’ & Artists’ Yearbook, either by buying it from a good bookshop or borrowing it from a library. After studying this book and identifying the category that your book falls into, send the details to publishers and agents who deal with that category.
The Writers’ & Artists’ Year Book gives full information on what documents to send and how to send them, but don’t restrict yourself to just one publisher or agent at a time and don’t expect replies in a hurry.
At the same time, send a synopsis of your book to any publishers mentioned in the newspapers who have published books on a similar subject to yours.
Now that we are in the Electronic Book age, see my publisher's conditions HERE and give him an opportunity to publish it.
How can I find a publisher or agent for the novel I am writing?