Some don't aspire to write books, and in an age of e-publication, this may not be as unusual as it sounds anymore. But for writers who enjoy having their work collected as a book, professionally, it is helpful to have some demonstrable credits and proof of prior publication before going to a publisher.
There's a few successful people who have never published in a journal before and knocked it right out of the park on the first try, but they're few and far between. So for the rest of us working writers, some basics:
1) Find a few journals and magazines to see if your writing would be of interest to that journal. And read them.
If they don't publish poetry already, the odds are highly unlikely your poem is going to change their mind. It's sad, but true, just as it's also unlikely that Bay Area Mothers Magazine is going to accept your poem about Swinging at the Hell's Angels Lemon Party with a Rusty Trombone in Steamin' Cleveland.
For a good guide to current journals around the world, check out: www.newpages.com
2) For poems, pick out a selection of three that demonstrate different styles and topics. If they don't like one poem, chances are reading two others about the same thing in the same voice and tone aren't likely to get you any further. Use a good format- avoid 'funky fonts' or too much avant garde presentation unless the journal really suggests that's what they like.
3) Use a basic cover letter. This will introduce your bundle:
* Who you are
* How many poems that you're submitting, and assure them that they've never been published before.
* The titles of the poems included.
* Provide a bio about yourself: One or two sentences about where you live, what you do for a living, a few other publication credits or awards and a way to read more of your work.
* Thank them for their time, and give them a way to contact you if they have any questions. E-mail addresses are usually sufficient these days.
Most journals accept e-mail submissions. Some don't accept attachments. Figure out what they prefer from their submission guidelines.
Submit once or twice a year. Don't ever bombard them with tons and tons and tons and tons of poems.
Over time, you'll want to diversify your poetry credits anyway, and poems published in one journal really shouldn't be submitted to other journals unless they specialize in reprints. This keeps copyright protections in the clear for everyone.
My other advice, after years of trial and error is don't submit to journals that ask for money to review or print your work.
Understand that most journals don't pay for your work either, and those that do rarely pay more than enough to buy a small cup of coffee. That's just the field at this point. Poets have to do this because they love to do it. To adapt the Harley Davidson riders motto: Live to Write, Write to Live.
As an FYI: As of 2010, the average book deal for poets seems to net you a flat $1,000. Some get a little more with larger presses but I wouldn't count on those too much. As a poet, when we're creating, one never expects to give up the day job.