I've heard that poetry is all about how you break the lines, and if you feel enthusiastic enough, rhyme.
Not quite...not at all, actually.
No one can really define a poem. In a collection of poetry, for instance (or maybe it was a magazine, originally; I've only seen it in anthologies), Charles Bernstein once had a page that contained only a single line:
This poem intentionally left blank.
Now, I find this hysterically funny and incredibly intellectually stimulating, but I refuse to think of it as a poem. Here, I am at odds with many critics. A poem can be, then, whatever you want it to be. But that doesn't make it a good poem.
Line breaks are essential, but there are no rules for poetry anymore. Wherever you feel a line should stop, you can stop it; there should be an intended effect on the reader for each line break (it isn't arbitrary), but there's no formula for it.
The same is true of rhyme and meter (the pattern of stressed an unstressed syllables in a line). In modern poetry, rhyme and meter are generally thrown out the window. It's not about being "enthusiastic" or not, but rather about what kind of poem you want to write. I personally write poetry with strict rhyme and meter (Robert Frost said that writing a poem without rhyme and meter is like playing tennis without a net--it's a good analogy, I think), but you should let your intention with each poem help inform the style/structure.
The most essential thing to remember, in my opinion, is this: poetry is not just about letting your emotions out. If you want to write poems as a way of venting your anger/frustration/sadness, that's fine; write them in your journal, and maybe share them with a friend or loved one if you want. But do not expect them to be poems of real literary worth--they are of personal value, surely, but that doesn't make them of value to others. True poetry, to paraphrase Wordsworth (I'm full of quotes today), is the result of extreme emotion recalled in a period of tranquility. Poetry has an emotional source, certainly, but it is organized, edited, and shaped. There must be an intellect guiding the emotion, that is, not a simple outpouring of angst.
