"I Am" is a great example of iambic pentameter, the most common meter in English poetry. Even the poem's title sounds like iamb. Anyway, you can read all about it here, but we'll give you the basics really quick. There are five beats (penta-) or feet; each foot is an iamb, which contains an unstressed syllable followed by a stressed syllable. So, an iamb sounds like da-DUM. (Say word "allow" out loud and you'll hear an iamb.) That's it. Piece of cake, right?
As an example, take line 4:
They rise and vanish in oblivion's host.
Really quick here, you should pronounce "oblivions" like "ob-liv-yuns" (it rhymes with "Funyuns," those super-delicious, onion-looking chips). As you can see, this is a very neat line of iambic pentameter: da-DUM, da-DUM, da-DUM, da-DUM, da-DUM.
Occasionally, however, Clare will change things up. Here's line 1:
I am yet what I am none cares or knows.
Notice that there are two feet ("beats") that contain two stressed syllables ("I am"). This is called a spondee, and it's a very emphatic beat. It's almost like the speaker is screaming out "I AM!!!" The double stress is very noticeable, especially at the beginning of a line, and it's the speaker's way of really hammering home the fact that he exists. See, and you thought meter was just for decoration!
Okay well that pretty much sums up the meter, but what about the poem's organization? Well, it's composed of three rhyming stanzas of six lines each (a six-line stanza is called a sestet, btw). The rhyme scheme in the first stanza is ABABAB, but the two other stanzas rhyme ABABCC. They both end up with two successive lines that rhyme, which is called a couplet.
So, why the change? For one thing, it's cool to change things up. It's a way of saying "Hey, look I can do this too!" More importantly, though, the inclusion of a final couplet in the poem's last two stanzas is a sign of harmony. It's the speaker's ways of suggesting that, despite his despair, he is starting to find some balance in his life. In the final stanza, for example, he's definitely a little more hopeful as he yearns for a peaceful and quiet death. In that way, the way the poem is put together goes hand in hand with the content of the words themselves. Nifty, huh?