The first three lines of the poem are all regular iambic tetrameter. This means that each line has four feet, each foot containing two syllables, one unstressed (or unaccented) syllable followed by one stressed (or accented) syllable. One way that you can begin to figure this out is by looking at the words that have more than one syllable and trying to work out which of their syllables are stressed: for example, "nothing" is stressed as NUH-thing and not nuh-THING, and "obey" is oh-BAY and not OH-bay. This makes it easier to scan the rest of the lines that those words are on because now you have a place to start.
That fourth line is a little trickier, though it is still written in iambic tetrameter, and it all has to do with pronunciation of the word "power." Technically, you are right, the line has nine syllables, but ask yourself, when you read the line, how do you pronounce "power"? I don't hear it stretched out into two fully pronounced syllables (POW-er), but rather, it sounds more like one accented syllable: POW'R. Poets sometimes manipulate language in this way: "fire" is another word that, in verse, can sometimes be pronounced as one syllable and other times two. When you read "power" as one syllable rather than two, the line retains its regular meter.
As the other commenter notes, the rhyme scheme is aabbcc, and so on (these rhyming pairs of lines are called rhyming couplets).
The rhyme scheme for this poem is "aabb".
The first three lines of the poem are all iambic tetrameter; however, the last line is also tetrameter, but the problem is that it has 3 iambic feet with 1 anapest foot thrown in the middle. That's why it's confusing. The last line does not follow a regular metric foot pattern.