There are several steps to getting work published.
As Svaha says, above, if you are producing poetry, you want to look at getting individual pieces published before you shoot for a collection, and the same is true of short stories.
However, there are certain rules which apply whatever work you are submitting, and wherever you are submitting it.
Make sure your work is ready.
Edit, edit, edit. Check spelling, check grammar. No allowances will be made for sloppy work -- it shows a lack of care. The editor's job at a publishing house is not to correct your errors but to suggest changes that will make the book more saleable. Before you send anything make it perfect.
Pick your market carefully.
Whether you are submitting a poem or a story to a magazine, a novel or a biography or whatever, you need to send it to the right place.
Check that the publisher you have selected does, in fact, publish your kind of work. If it's a magazine, read a couple of issues. If it's a publisher, check their catalogue.
Consider submitting to periodicals who don't pay but have a good reputation, as this gives you a publishing history.
Find a Name.
Once you have decided on your market, do some research, and find out the name of the editor who deals with your kind of work. It's good manners, and puts them on-side.
Follow guidelines to the letter.
Most publishers ask for work to be printed on one side of the paper, double spaced, paragraphs indented with no space between paras. No matter how old fashioned you think this is, do it. If you don't, your work will probably be rejected without reading it. The same goes for any other guidelines -- these may relate to page numbering, where and how your name should appear on the manuscript, format of files by email or disk. If you ignore the rules, you can kiss goodbye to your chances of seeing your stuff in print.
If a publisher asks for queries, don't send a full manuscript -- send a chapter or two and an outline for the rest of the book.
Put in a covering letter
This should contain a synopsis of the work you are attaching, your contact details, your publishing history, and, where relevant, your qualifications for writing on the topic you have chosen. Nothing else. The publisher really isn't interested in your three lovely kids, and whether you collect carnivorous plants -- at least, not until they have accepted your masterpiece and want to write an author bio.
Do not, and I mean ever, open your letter with "I don't expect this is good enough, but..." If you don't think it's good enough to publish, why are you sending it? (And yes, I have received letters that open that way, in my editor's role.)
Unless it is explicitly stated that the publisher accepts simultaneous submissions, only send to one publisher at a time.
Good manners again -- this isn't like looking for a job, you can't send out multiple applications. If you were to send your book to two publishers and receive an offer from both,you would almost certainly have both offers withdrawn and be blacklisted into the bargain. It just isn't done. Frankly, publishers see more publishable books than they will ever need -- they might like yours, but they don't need it.
Include return postage for your manuscript
This will at least guarantee you get your work back, and some kind of response. If you don't do so, they will assume you don't want your book back even if they decide against publishing.
Be prepared to wait.
Publisher slush piles are huge. You cannot expect a response within a matter of weeks, and it could easily be a year before you get a yes or a no. Once you have sent your manuscript, start working on the next, and accept you are in for a long wait. After about three to six months, you can reasonably send a polite reminder, but don't hassle, unless you want to get a reputation for being troublesome.
Everyone receives them. They may be a form letter, or a personal one with reasons, but pretty much every successful writer has a collection of them. Even Harry Potter was rejected before it found a publisher. If you can't stand rejection, don't even think about becoming a writer.
Above all, be patient, and keep optimistic-- the difference between published and unpublished writers is perseverance.